We have recently demonstrated that human thymocytes can be induced to express interleukin 2 (IL-2) receptors and to synthesize IL-2. The present study shows that relatively immature T6+ human thymocytes as well as the more mature T3+ thymocytes could be induced to express functional IL-2 receptors when activated with either Concanavalin A (Con A), Con A and 12-O-tetradecanoylphorbol 13-acetate (TPA) or IL-2 in combination with Con A or TPA. The phenotype of the common, immature thymocyte was identified by the binding of either fluorescein isothiocyanate (FITC)- or phycoerythrin (PE)-conjugated monoclonal anti-T6 (OKT 6) antibody and of mature thymocytes by the binding of monoclonal anti-T3 (OKT 3) antibody. We also observed that the expression of the T3 antigen on thymocytes, freshly isolated from thymic specimens obtained in the course of cardiac surgery of infants and children, was greater than 40% in 14 of 18 donors and that thymocytes co-expressed the T3 and the T6 antigen as determined by dual colour cytofluorometry. In thymocytes activated in vitro the expression of IL-2 receptors, determined by dual colour cytofluorometry with the PE-conjugated monoclonal anti-human IL-2 receptor antibody (PE anti-IL-2 R), was detected by the second day of induction in both immature T6+ and mature T3+ thymocytes. T6+ thymocytes proliferated in response to IL-2 and persisted in cultures for the duration of the study (18 days) and continued to express IL-2 receptors.