Distribution and modulation of rabbit lymphocyte phytohaemagglutinin acceptors and concanavalin A acceptors during activation of rabbit lymphocytes have been examined by electron microscopy. Two types of cell surface acceptors have been tentatively identified, lectin binding acceptors that do not modulate, and receptors that are endocytosed when blast transformation is stimulated. All of the cells have binding acceptors for both lectins. Endocytosis correlates with early blast transformation and serves as an early marker for lymphocyte activation. When examined after 24 h of culture, those cells that undergo blast transformation contain endocytosed lectin receptors, whereas small untransformed cells do not. Capping prior to endocytosis is rarely observed. The mechanism whereby the signal for transformation is maintained after the reaction of lectin with cell surface receptors and transposed to the nucleus is not known. Although we conclude that endocytosis is an early event required for cell activation, it is possible that endocytosis is secondary to other activation events. By evaluation of sequential endocytosis, individual rabbit lymphocytes that endocytose only concanavalin A, only phytohaemagglutinin, both concanavalin A and phytohaemagglutinin, or neither lectin, have been identified.