A technique is described for the detection and enumeration of cells producing antibody to O-somatic antigen specificities 1, 4, 5 and 12 of Salmonella typhimurium. Using this technique, a difference in the immune response has been demonstrated between Swiss White and the BALB/c strain mice, when both these strains are injected with a standard dose of acetone-killed S. typhimurium C5 vaccine. BALB/c mice fail to respond to antigen 5 and their response to the 1, 4 and 12 O-somatic antigens, while reaching the same magnitude, is less prolonged than that in Swiss White mice.