The influence of two interleukin-2 (IL-2) preparations on the proliferative response of normal lymphocytes to phytohemagglutinin (PHA) was examined. A recombinant IL-2 (rIL-2) and an IL-2 containing conditioned medium (LyIL-2) markedly enhanced 3H-thymidine incorporations at low PHA concentrations, whereas at optimal mitogen concentrations, this effect was marginal. In lymphocyte cultures of 3 patients after autologous bone marrow transplantation (ABMT), exogenous IL-2 augmented PHA-induced stimulations. Moreover, a dose-dependent increase of the 3H-thymidine uptake was observed in unstimulated cultures of all normal donors and patients. Combined autoradiography and surface marker analysis allowed to identify cells spontaneously proliferating in the presence of exogenous IL-2. Comparison of phenotypes of these cells revealed pronounced differences between a normal donor and a patient after ABMT.