Human peripheral blood T lymphocytes are stimulated to grow and divide by lectins such as concanavalin A (Con A) and Phaseolus vulgaris phytohemagglutinin (PHA), as well as a few anti-T cell monoclonal antibodies. The latter antibodies recognize the T3 antigen. It has been suggested previously that PHA and Con A mediate T cell growth by interacting with T3. However, as reported in this study, affinity chromatography on immobilized lectins, and immunoprecipitation by lectin plus anti-lectin antibodies showed that T3 binds Con A but not PHA. Fab fragments of a monoclonal antibody against T3 (namely Leu-4) inhibited T lymphocyte proliferation induced by T3 antibodies and Con A, but not by PHA. Nevertheless, co-capping experiments performed with fluorescein-labeled lectins and rhodamine-labeled T3 antibodies showed that T3 co-caps with Con A and PHA receptors, although the co-capping with PHA was incomplete. Since the T cell receptor for antigen (Ti) has been shown to co-cap with T3 on the cell surface, we reasoned that PHA induced capping of the T3 antigen by interacting with Ti. A disulfide-linked heterodimer comprising subunits of about 49 000 and 41 000 mol. wt. that resembled the Ti molecule was detected in PHA-anti-PHA immunoprecipitates of various surface- and biosynthetically-labeled T cells, by two-dimensional (nonreduced vs. reduced) sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis. The results suggest that PHA triggers T lymphocytes by interacting with the carbohydrate moieties of Ti and imply that T lymphocytes can be stimulated by mitogens via at least two different cell surface molecules (Ti and T3).