The capacity of the heavy cations Pb, Ni, and Zn to modulate murine in vitro lymphocyte responses was examined. Pb and Ni (100 microM) were shown to enhance the in vitro plaque-forming cell (PFC) response to sheep red blood cells while 100 microM Zn had inhibitory effects. Each metal was able to stimulate the proliferation of murine splenocytes as determined by [3H]thymidine incorporation and autoradiography. The enhancing effect of the metals on the PFC response was observed whether the results were expressed on a per culture or a per cell basis, indicating an actual increase in B-cell differentiation. Both the PFC response and the proliferative response were shown to be sensitive to the type of medium employed (M-199 gave optimum results) and to the presence or absence of 2-mercaptoethanol. As in autologous mixed-lymphocyte responses peak proliferation occurred after Day 5 in culture, was cell density dependent, and required the presence of both T cells and Ia+ cells. Treatment of splenocytes with anti-Thy-1.2, anti-Lyt-1, or anti-L3T4 plus complement completely abrogated the proliferative response, indicating that a Lyt-1+, Lyt-2-, L3T4+ T-cell was required for the induction of proliferation. The data are consistent with the hypothesis that the metals are capable of modifying the immune response directed at self either by directly altering self constituents (class II) or by modulating the autologous T-cell response.