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Equine antisera to human adenovirus types 1 to 33 were prepared and evaluated by hemagglutination-inhibition and serum neutralization tests. Detailed data on the potency and purity of the immunizing antigens were tabulated as one means of evaluating the antisera. Most of the 52 hemagglutination-inhibition and 25 serum neutralization major or minor heterotypic responses among the equine antisera were observed at similar levels in previous studies with rabbit antisera and appeared to represent genuine antigenic relationships among the human adenoviruses. Equine antisera to human adenoviruses 1 to 33 and a similarly packaged normal horse serum served as lots of fully tested sera for definitive typing of isolates and as reference standards for evaluating other antisera.

During the Hong Kong influenza epidemic of 1968-1969 the attack rate among persons born prior to 1890 was about one-third the rate among persons born after 1899. During an Asian influenza epidemic in the same community two years earlier there was no apparent age effect. Most persons born before 1890 had detectable pre-epidemic antibody against A/Hong Kong/68 whereas most persons born after 1899 had no detectable pre-epidemic antibody. The lower attack rate in the older age group is consistent with the hypothesis that influenza A viruses with similar or identical hemagglutinins can reemerge over time and that remote previous experience with the same or a similar strain can be protective. Also, no age group born after 1890 was found to have a high prevalence of antibody against A/England/72 or A/Dunedin/73, the successors to A/Hong Kong/68, indicating that recycling of hemagglutinin antigens is not strictly ordered.

When influenza virus was mixed with antisera to its surface subunits before inoculation of cell cultures, anti-hemagglutinin antibodies neutralized infectivity but anti-neuraminidase did not. When the antisera were added after infection of cell cultures, anti-hemagglutinin and anti-neuraminidase antibodies were equally effective in reducing virus titers in culture fluids. Decreased virus titers were not due to interference of antibody with assay and were not accompanied by a reduction in the synthesis of hemagglutinin and neuraminidase subunits. Both antisera also effectively prevented in vitro virus spread. Inhibition of virus release by neuraminidase antibody appeared unrelated to its antienzyme property. Hydrolysis of N-acetyl neuraminic acid residues of infected host cells proceeded unimpaired in the presence of subunit antisera. Anti-hemagglutinin and anti-neuraminidase antibodies may act to prevent virus release by binding newly formed virus subunits to each other and to anti-genically altered cell membranes.

The antigenic relationships among the neuraminidases of influenza A strains from 1957 to 1973 were examined by postinfection application of neuraminidase antisera. This procedure causes inhibition of virus spread and apparent neutralization. Neuraminidase (apparent) neutralization and neuraminidase inhibition tests with chicken antisera gave similar results. Neuraminidase inhibition tests were more discriminating than neuraminidase neutralization tests when rabbit and goat antisera were used. Antibody absorption studies revealed that the neuraminidase, like the hemagglutinin, may possess two kinds of antigenic determinants, which can give rise to "common," or "cross-reacting," and "specific" antibodies. "Specific" antibody appears to be more effective in the inhibition of enzyme activity than in the inhibition of virus spread.

Single radial diffusion (SRD) tests for antibodies to influenza type A hemagglutinin, neuraminidase, nucleoprotein, and matrix protein antigens were compared with conventional hemagglutination inhibition, neuraminidase inhibition, and complement fixation tests. Sera used in this study were obtained in 1968-1969 from volunteers before and after vaccination and before and after an ensuing epidemic of Hong Kong influenza. The SRD test compared favorably with conventional tests for assessment of vaccine- or infection-induced rises in antibody titers to influenza type A viruses. Little linear relationshiip was seen between zone areas with SRD and titers with conventional tests, suggesting that the SRD test may detect antibody of different quality or specificity. The SRD seemed equal to the hemagglutination inhibition test for predicting susceptibility to influenza. SRD is a simple test for the recognition of antibody to various antigenic components of the influenza virus and could prove to be a valuable epidemiological tool.

Erythrocytes sensitized with purified neuraminidase (Hong Kong) antigens were used for assay of influenza A neuraminidase antibodies. The neuraminidase indirect hemagglutination test was equal to the neuraminidase hemagglutination-inhibition (enhancement) test and appeared to be better than the neuraminidase inhibition test for detection of fourfold or greater antibody rises in paired sera from influenza patients or vaccinees. It was better than both tests for detection of neuraminidase antibody. The neuraminidase indirect hemagglutination test is simple to perform and has the advantage of direct antigen-antibody assay.

A simple procedure for the production of large volumes of purified adenovirus group-specific complement-fixing (CF) (hexon) antigen by selective adsorption to and elution from CaHPO4is described. Results of immunodiffusion tests, electrophoresis, electron microscopy, and tests for hemagglutination and infectivity indicate that the purified antigen consisted of a single virus component (hexon). The purified product contained little host materials. Unlike the crude virus harvest usually employed for serodiagnostic CF tests, the purified antigen demonstrated no anticomplementary activity and did not develop such activity during storage. The purified antigen was equal to or slightly more sensitive than crude virus harvests for serodiagnosis of adenovirus infections.