The authors investigated the ability of various lymphocyte subpopulations to synthesize substances with thymosin-like activity under the influence of thymostimulin, a preparation of active thymic factors. Experiments carried out on T- and B-mice revealed that only T-lymphocytes synthesized thymosin-like substances in response to thymostimulin administration. Thus induced substances differed significantly from those which ensure the serum activity of normal animals in that their thermostability is by far greater. The feasibility was also explored of inducing by thymosin the in vitro synthesis of substances with thymosin-like activity in cells derived from a variety of lymphoid formations of thymectomized mice. The serum of thymectomized lethally irradiated recipients of lymph node and spleen cells pretreated with thymostimulin was found to contain, 4 h after the transfer, relatively high levels of substances with thymosin-like activity. Contrariwise, the pretreatment of bone marrow cells with thymostimulin resulted in declined levels of substances with thymosin-like activity in recipient sera as compared to those in B-mice which were given untreated bone marrow cells. The incubation of bone marrow cells in a medium containing substances with thymosin-like activity caused a decrease in that activity. Thus, the biologically active thymic factors were shown to induce the synthesis by T-cells of substances with thymosin-like activity.