Unpurified lymphocytes from spleen, lymph nodes, thymus, and T and B cell-enriched fractions of spleen cells from NMRI mice as well as lymphocytes of nude mice were cultured and stimulation of the lymphocytes by DEAE-D, Con A and LPS and combinations thereof was measured by incorporation of 3H-thymidine. It was demonstrated that DEAE-D is a rather weak mitogen for mouse lymphocytes, acting apparently on B cells as well as on T cells. Frequency and intensity of lymphocyte stimulation by the T cell mitogen Con A and the B cell mitogen LPS was much better. When combinations of DEAE-D with Con A or LPS, respectively, were used significant enhancement of lymphocyte stimulation occurred, as compared to the mitogens and DEAE-D alone. This enhancement was more prominent with the DEAE-D/Con A combination than with the DEAE-D/LPS combination, and more pronounced with the T cell-rich lymphocyte preparations than with the B cell-rich suspensions. The results are discussed and it is concluded that DEAE-D enhances both T cell and B cell functions, however, T cell functions are favoured. This enhancement is assumed to be mediated by a membrane effect of DEAE-D, and to be responsible for th immunological adjuvant effect of DEAE-D.