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A method has been devised for inhibiting the normal enteric flora, permitting long term asymptomatic enteric infections of mice and guinea pigs with streptomycin-resistant strains of Shigella flexneri or Vibrio cholerae. Introduction of a streptomycin-resistant strain of E. coli into the intestinal tract of experimental animals resulted in a rapid elimination of the enteric pathogens studied. No in vitro production of antibiotic substances by this coli strain could be demonstrated. Active and oral passive immunization did not noticeably influence the number of Shigella or Vibrio organisms recoverable from the feces of infected animals.

The results presented indicate that blocking curves of decreased slope are obtained with the Talmage test when the labelled antibody eluate contains a portion of antibody which is not represented in the unknown serum. The blocking curves of late antisera had steeper slopes than those of early antisera when tested against labelled antibody eluates prepared from late antisera. This situation was reversed when labelled eluates from early antisera were used in the Talmage test. The findings were interpreted to indicate that early antisera did not have a full complement of antibodies, directed against all surface antigens of the bacterial cell.

Passive oral immunization with O-antiserum was found to protect guinea pigs against fatal enteric cholera. The number of vibrios recoverable from the intestines of infected animals was not noticeably reduced by active and oral passive immunization. Passive immunization by the intraperitoneal route was not protective even though high titers of circulating antibody were obtained shortly after injection. Oral administration of H-antiserum also had no protective effect. Introduction of E. coli into the intestinal tract increased the resistance of guinea pigs to fatal enteric cholera.

Studies of the relative combining capacities of different antisera to Vibrio cholerae and Shigella flexneri were carried out using the Talmage test. In this test the absorption of an I(131) labelled antibody preparation by antigen is blocked by the addition of unlabelled unknown serum. Sera from rabbits in the early stages of immunization ("early antisera") had a high agglutinin titer, but low combining capacity. Sera from hyperimmune rabbits ("late antisera") had higher combining capacities than early antisera, but similar or lower agglutinin titers. More antigen was needed to absorb the agglutinins from late antisera than from early ones of the same agglutinin titer, indicating that late antisera contained relatively more antibody. At high concentrations, sera from hyperimmune rabbits agglutinated the homologous antigen more rapidly than did early antisera, even though early sera had similar or higher agglutinin titers. Sera drawn after the anamnestic response to bacterial antigen had the characteristics of late sera; i.e., their combining capacities were high. The possible significance of these findings for the in vitro estimation of protective antibody is discussed.