Culture supernatants of mixed lymphocyte reactions (MLR) between AKR thymic nurse T cells (TNC-T) as responders and irradiated DBA/2 spleen cells as stimulators, induce significant proliferation of concanavalin A (Con A)-stimulated DBA/2 thymocytes. The culture supernatants of MLRs using extra-TNC thymocytes (ET) as responders do not show this activity, and the addition of thymus epithelium did not enable a response to take place. Again, unlike ET, TNC-T cells could also give a positive allogeneic response. Irradiated AKR TNC-T cells stimulated allogeneic BALB/c spleen B cells to produce immunoglobulin. These observations indicate that cells in the TNC-T pool are more mature than those in the ET pool.