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The present four-center collaborative study was undertaken in an attempt to define the best vaccine and/or vaccination procedure for use in areas of the world that are free of smallpox. The study was designed to compare the effect of different vaccinial strains, viral concentrations, and routes of administration on the morbidity and antibody response associated with primary vaccination and standard challenge revaccination. Primary vaccinations were performed on 1,585 children; 49.6% of the children were vaccinated by the percutaneous route, and 50.4% received vaccine subcutaneously. The overall age and sex distributions of percutaneous and subcutaneous vaccinees were comparable, but there were marked differences in participants among the four study centers. Vaccines in Kentucky had a greater mean age; the greatest number of Negroid children were enrolled in St. Louis, and more of them were vaccinated by the subcutaneous route; and the dropout rate was much greater in San Diego and Colorado. An analysis of comparative interlaboratory serologic procedures with the use of 20 coded duplicate samples of serum revealed good agreement in the hemagglutination-inhibition test; results of neutralization tests had greater variability of mean titers. On duplicate samples of serum from study participants there was generally good correlation between each of the four study centers and the Center for Disease Control's reference laboratory in titers of hemagglutination-inhibiting antibody. In contrast, 38% of the neutralization titers determined at the four study centers were greater than or equal to 0.67 log10 higher than the respective titers noted at the Center for Disease Control. [ABSTRACT FROM AUTHOR]