AbstractThe surface antigenic phenotype of peanut lectin agglutinin (PNA)‐separated, high‐density (“immature”) thymocytes which are the target for the T cell growth factor (TCGF) enabling this otherwise unresponsive cell population to mount a strong proliferative response to concanavalin A (Con A) has been studied. The results obtained indicate that (a) the major population of cells bearing the typical phenotype of immature thymocytes,i.e.low content of H‐2, high content of Thy‐1, TL, Lyt‐1 and Lyt‐2 antigens, is unable to respond to Con A in the presence of TCGF, and (b) that the responsive population constitutes a minor fraction (about 15%) of PNA+, high‐density thymocytes with surface antigenic phenotypes similar to those of mature T cells,i.e.high content of H‐2, low content of Thy‐1 and differentiated expression of Lyt antigens. Such a unique combination of properties and the presence of TL antigen on some members of the responding population suggest that it contains cells at intermediate stage(s) of differentiation between immature t