• Groups of children who had been given different doses of a poliomyelitis reference vaccine A in the spring of 1955 were all reinoculated in the spring of 1956 with the same dosage of vaccine J. Quantitative studies demonstrated both the persistence of the antibody over the course of a year and the increase in titer in response to the third dose given at this time. To secure an optimum response to the booster injections it was necessary to give an adequate dose in the primary antigenic stimulation. After the booster injections the antibody titers equaled or exceeded those found in a group of convalescents from recent type 1 paralytic poliomyelitis. The nonliving, nonmultiplying virus antigen reproduced the serologic effects of naturally acquired infection-immunity. Seven lots of commercially prepared vaccines administered to 186 subjects, compared with five lots of laboratory-prepared vaccine in 115 subjects, proved to be equally satisfactory as to potency and uniformity in raising the antibody titer. In 4,617 subjects who received three inoculations, only 224 had antibody at a level of 1:64 for all three types before inoculation, but 4,548 had antibody at this level for all three types after the third inoculation. Six lots of commercially prepared vaccine were used in a study of 214 subjects as to the advantages of a second inoculation, and the rise in titer produced by the second inoculation was clearly demonstrated for each lot, so that a low proportion of response to the first dose was converted to a high proportion of response by the second dose.