Pittman, Margaret (National Institutes of Health, Bethesda, Md.), and Howard J. Bohner. Laboratory assays of different types of field trial typhoid vaccines and relationship to efficacy in man. J. Bacteriol. 91:1713-1723. 1966.-Antibody responses of rabbits to H, O, and Vi antigens did not differentiate vaccine K (acetone-killed and dried) from vaccine L (heat-phenolized and dried) relative to human efficacy. A mouse protection assay in which intraperitoneal vaccination and challenge suspended in mucin were used showed vaccine K to be 3.69 times more potent than vaccine L; with subcutaneous vaccination, vaccine K was only 0.78 as potent. With the challenge suspended in saline, the effect of the route of vaccination was accentuated. An old U.S. reference vaccine, heat-phenolized, induced the same types of response as vaccine L. In the assay with intraperitoneal vaccination and mucin-suspended challenge, the potency of vaccine K relative to vaccine L and the potencies of Polish vaccines, P, N, and T, relative to vaccine K were directly correlated with the efficacies of the vaccines for man, as reported for the recent World Health Organization cooperative field trials in British Guiana, Yugoslavia, and Poland. This assay gave a high potency for alcohol-treated vaccine V relative to vaccine L, but the values did not reflect relative efficacy in the USSR field trial; the subcutaneous vaccination assay more closely reflected their human efficacy. An analysis suggested that vaccine K had a mouse protective factor not present in vaccine L and that vaccine V may have had a third factor. The influence of a few variable factors on the assay with intraperitoneal vaccination and mucin-suspended challenge was studied briefly.