In the present work the effect of Isoprinosine on the mitogenic responses of T and B lymphocytes has been studied. We have found that Isoprinosine can enhance in vitro the response to Concanavalin A. This enhancement was more apparent in cell cultures showing an initially low blastogenic response. In low responses artificially induced by treatments in vivo with cyclophosphamide, our results indicate that Isoprinosine, administered in vivo, does not enhance the response to Con A of treated mice. However, addition of Isoprinosine (75 micrograms/ml) to cultures of spleen cells from mice previously treated with cyclophosphamide enhanced the suppressed response up to normal levels. Neither in vivo nor in vitro Isoprinosine treatments increased the response of lymphocytes to lipopolysaccharide, but usually inhibited the blastogenesis of B cells.