Cells isolated from the spleen of an adult rabbit and mixed with an antigen in vitro (S. paratyphi B) form antibodies when injected intraperitonealy in five-day-old rabbits. At this age, young rabbits do not react to a transferred antigen by the formation of antibodies. No difference was found in the degree of antibody formation on the injection of cells from a normal rabbit, mixed with the antigen, cells from the spleen of a rabbit immunized with a non-specific antigen (guinea-pig serum) or cells immunized with a specific antigen (S. paratyphi B). Antibody formation is made possible by an optimal quantitative relationship when mixing the cells and the antigen in vitro. In a corpuscular antigen, two micro-organ-isms of S. paratyphi are added to one spleen cell. Large doses of the antigen depress the formation of antibodies. If isolated cells are irradiated with a dose 860 r, antibody formation is not depressed, whereas irradiation of cells with 1000–1200 r completely stops the formation of antibodies. The transfer of spleen cells from an adult rabbit does not induce an active response to the antigen in young rabbits. The participation of the recipient (the young rabbit) in antibody formation is regarded as passive, as a suitable medium for the transfer of cells. Cells injected into an organism survive; antibody formation can be induced by injecting the young animals with the antigen 24 and 72 hours after the injection of plain washed spleen cells. Antibodies are formed, therefore, following the intraperitoneal injection of cells and injection of the antigen into the blood stream. On mixing the cells with the antigen at a temperature of 37° C, the effective amount of antigen rapidly becomes bound by the cells. Washing of the remaining antigen following incubation does not stop antibody formation.