Peritoneal exudate lymphocytes obtained from immune guinea pigs and cultured for 1 wk on antigen-pulsed autologous macrophages were tested for their ability to bind to fresh antigen-pulsed autologous macrophages or to macrophages pulsed with an irrelevant antigen. Up to 30% of the lymphocytes bound to macrophages bearing the relevant antigen whereas only 2-5% remained nonspecifically bound to bacrophages after vigorous washing. Specific binding was observed in cultures as early as 1 h. Analysis of the kinetics of binding suggests that the observed nonspecific binding is not a step in specific binding. The possibility that weaker antigen-independent association between lymphocytes and macrophages precedes specific binding cannot be excluded. No evidence was obtained that serum antibody adsorbed to the macrophage or T cell plays a role in this cell interaction or that the T cell can bind antigen directly. The observed specific binding probably represents the initial event in stimulation of T lymphocytes by antigen.