The proliferative responses to phytohemagglutinin (PHA) and to OKT3 monoclonal antibody of various human thymocyte subsets were studied. Unfractionated thymocytes are very poorly responsive cells, as assessed by both PHA and OKT3 stimulation. The mitogen responsiveness is confined to the T3-enriched (T3+) subset, the T6-positive (T6+) cells being almost completely devoid of proliferative capacity. The addition of exogenous IL-2 increases the proliferative responses to PHA and OKT3 of both unfractionated and T3+ thymocytes. This implies that the endogenous IL-2 production by thymocytes is inadequate to fully support the intrinsic proliferative capacity of these cells. Even in the presence of an optimal amount of IL-2, T3+ thymocytes exhibit proliferative responses of lower magnitude as compared to those of peripheral T counterpart (PBT). These observations indicate that the maturative level attained by the T3+ subpopulation within the thymus is considerably inferior to that of T3+ fully immunocompetent peripheral T lymphocytes. Macrophages are activated to produce IL-1 following the stimulation by OKT3-pulsed T lymphocytes. T3+ thymocytes are markedly less efficient than PBT in inducing IL-1 secretion. These data suggest that T3+ lymphocytes within the thymus are relatively immature in terms of cooperative capacities with accessory cells. In conclusion, T3+ cells constituting the more mature intrathymic pool do not reach a complete functional differentiation as compared with the T3+ peripheral counterpart.