An antibody response showing characteristics of a "true" primary response was readily induced in vitro with splenocytes from normal non-immunized animals cultured with strongly immunogenic Vibrio cholerae somatic antigens. Prior studies have shown that the response to vibrios in intact animals appeared to be a true primary response since no pre-existing antibody forming cells were present in non-immunized animals and the antigen induced response, following a lag of two days, resulted thereafter in a rapid stepwise increase in the number of specific PFCs, reaching a peak at 12 to 14 days. Using the Marbrook culture system for antibody formation a readily detected immunocyte response to vibrios was induced with splenocytes from normal non-immunized animals. No background antibody forming cells developed to the organisms without addition of vibrios in vitro. After in vitro immunization with 10(6) bacteria significant numbers of IgM PFCs appeared with a peak response on days 7 to 8. Splenocyte cultures from mice primed earlier with vibrios developed a marked secondary response, with appearance of both IgM and IgG PFCs. Large numbers of both classes of PFCs developed, with peak responses on days 6 to 7, similar to the "primary" response to sheep erythrocytes. However, significant numbers of PFCs to vibrios developed in cultures of vibrio-primed cultures even in the absence of vibrios during the 12 day culture period. The availability of a completely in vitro model system to induce a true primary immune response without the complication of pre-existing background antibody forming cells will be of value for further studies concerning various cellular pathways and interactions during the immune response to small amounts of strongly immunogenic bacterial antigens.