A protocol by which a human spleen cell culture could be subjected to both a primary and a secondary type immunization in vitro has been developed. Primary immunized cultures required the addition of autologous fresh cells at the time of antigen re-exposure for generation of optimal antigen-specific IgG responses. Antigen re-exposure (secondary stimulation) was done 10 days after initial immunization. Subsequent responses were characterized by novel or greatly enhanced antigen-directed IgG responses compared to primary antigen stimulation. Re-exposure of antigen to 10-day old human spleen cell cultures, without addition of fresh cells, did not result in measurable responses. Lethal irradiation of the fresh cell component of a restimulation culture showed that the IgG responses were derived from the original culture and not the fresh cells. The presence of antigen during both the primary and the secondary immunization was a prerequisite for induction of antigen-directed IgG responses, as spleen cell preparations that had been cultivated without antigen, during the primary stimulation period or the secondary stimulation period or both, did not show antigen-dependent IgG responses after the secondary cultivation. Induction of IgG responses by restimulation was supported by IL-2, but only when IL-2 was added to both the primary and the secondary culture according to a strict protocol. IL-4 added during the second antigen exposure supported antigen-directed responses, whereas IL-4 added during the initial antigen exposure abrogated all antigen-dependent responses.