AbstractThe immediate chemiluminescence (CL) response to concanavalin A (Con A) of rat thymocytes is enhanced 10 to 20‐fold when the cells are preincubated in serum‐free medium for 5–20 h. During this period, multiple encounters between lymphocytes and macrophages occur which morphologically appear as rosettes or grape‐like cell aggregates. Addition of bone marrow‐derived macrophages increases the number of cell aggregates and also the CTL response to Con A. Paradoxically, removal of macrophage‐containing cell aggregates after cocultivation leaves a pure thymocyte population which strongly responds to Con A with CL.Our results confirm that macrophage‐depleted “competent” lymphocytes are capable of CL and, furthermore, that “competence” is gained during cocultivation with macrophages. We are therefore convinced that measurements of macrophage and lymphocyte CL are a powerful tool for further elucidation of lymphocyte differentiation and of interactions between macrop