The proliferative response to peripheral blood mononuclear cells to phytohemagglutinin was enhanced at 40 degrees C when compared to those at 37 degrees C. This change in proliferative response was not affected by depletion of macrophages. However, T and non-T subsets of lymphocytes, separated by sheep red-blood cell rosetting technique, differed in their responsiveness to mitogens at elevated temperatures. The replication of DNA in T cell by phytohemagglutinin was enhanced at 40 degrees C. The kinetics of proliferative response with regard to dose of mitogen or duration of culture was similar at both temperatures. The response of non-T cells (B-cell enriched fraction) to pokeweed mitogen was unaffected. Similar result were obtained in experiments with lymphocytes from tonsil and thymus. Coculture of purified T and non-T cells from the same individual showed that responses to mitogens were synergistically stimulated both at 37 and 40 degrees C. The effect of elevated temperatures on the response of lymphocytes to concanavalin A was found to be variable.