A comparative study of in vitro induced lymphocyte transformation with cultures of peripheral blood cells from rabbits immunized to favor delayed sensitivity or antibody production was done. Cultures made at intervals during the animals' primary, secondary, or hyperimmune in vivo response to the C-RBC antigen were studied for the transformation phenomenon morphologically and by monitoring the incorporation of tritiated thymidine and uridine. The responses as judged by these characteristics indicate that the transformation process correlates with both types of distinct immune responsiveness. The appearance of large transformed cells with characteristic pyroninophilic cytoplasm and increased thymidine incorporation upon specific stimulation can be taken as evidence for immunologic activation. Animals minimally immunized to produce antibody alone or to exhibit delayed hypersensitivity without detectable humoral antibody incorporated less isotope into lymphocyte cultures than did either hyperimmune or boosted delayed sensitive animals. That these boosted animals were also exhibiting a single immune state was shown as definitively as possible by hemagglutinin titers and skin testing done at times of culture. Stimulation of cell cultures could be evoked with chicken erythrocytes or a urea extract but not with homologous erythrocytes. Cells cultured for prolonged periods in vitro and with varied concentrations of both the cellular and soluble forms of the antigen reflect differences in the transformation response of delayed and antibody-producing animals.