Highly purified lymphocytes were obtained from the lymph nodes of CFA-immune guinea pigs. Cultures of these lymphocytes did not exhibit proliferative responses upon stimulation with purified protein derivative (PPD), although they were quite responsive to stimulation with phytohemagglutinin. Addition of macrophages to these cultures resulted in marked enhancement of the PPD-induced lymphoproliferative response, but did not affect lymphocyte responsiveness to phytohemagglutinin. The ability of the purified immune lymphocytes to interact directly with soluble PPD was examined by incubating the cells for 1 hr at 37°C with various concentrations of PPD. The lymphocytes were then washed to remove unbound antigen and placed in culture. Addition of macrophages to cultures of antigen-pulsed lymphocytes did not result in the significant DNA synthesis, suggesting that the specifically immune lymphocytes were unable to bind and carry into culture sufficient antigen to allow for their subsequent activation. These results were not due to the induction of tolerance in the lymphocytes, since cultures of antigen-pulsed and untreated lymphocytes were equally responsive to continuous PPD when macrophages were added to the cultures, even when the antigen-pulsed lymphocytes had been exposed to high concentrations of PPD for 1 or 24 hr. In contrast, macrophages which were incubated for 1 hr at 37°C with a given concentration of PPD were as effective as the same concentration of PPD present continuously in macrophage-lymphocyte cultures in the induction of a lymphoproliferative response. Macrophages appear to play an obligatory role in the presentation of antigen to the immunospecific thymus-derived (T)-lymphocyte.