In vitro cultures of the peripheral blood lymphocytes of rabbits may be stimulated with phytohaemagglutinin, staphylococcal filtrate, antiallotype serum, or sheep anti-rabbit whole serum to synthesize protein, RNA and DNA as indicated by the incorporation of radiolabelled precursor substances into these products. A sequence of events found in all stimulated cultures characteristically shows protein synthesis followed by RNA synthesis, histologic blast transformation, DNA synthesis, and mitosis, with the complete sequence requiring 48 hours. All four stimulants induce essentially identical metabolic changes. Characterization of the proteins synthesized by lymphocytes in vitro has failed to demonstrate immunoglobulin synthesis by stimulated or non-stimulated cultures. It is concluded that the majority of proteins produced by peripheral lymphocytes stimulated in vitro are most likely cellular proteins related to the metabolic alterations necessary for mitosis. Absorption of sheep antisera to whole rabbit serum with rabbit IgG does not always remove the transforming capacity of the sheep antisera. Thus, it is likely that antibodies to proteins other than IgG present in the small lymphocyte may also be able to stimulate transformation. A possible common mechanism for the induction of lymphoblast transformation may be the ability of both specific and non-specific stimulants to react with protein constituents of the lymphocyte which may also be present in serum.