The immunological responses to sheep red blood cells (SRBC) were evaluated in mice in order to study the immunomodulating effects of an insoluble fraction of Corynebacterium granulosum, designated P40. A single intravenous (IV) injection of 50 micrograms or more of P40 was able to stimulate the reticulo-endothelial system, as measured by the spleen index. A single IV injection of 250 micrograms of P40, given seven days prior to IV sensitization of groups of mice with varying doses of SRBC, was able to amplify both humoral and cellular immune responses. Lower doses of P40 given subcutaneously (SC) were able to potentiate delayed-type hypersensitivity (DTH) to SRBC. Potentiation depends on the route, dose and time of the P40 injection in relation to the time and route of sensitization. A bimodal activity of P40 was observed: one peak was noted with SRBC were mixed with P40, and a second peak occurred when P40 was injected at the same site four to five days prior to immunization. Maximal potentiation was achieved when SRBC were introduced SC together with 5 micrograms of P40. DTH reached a higher level in animals immunized under the influence of P40; however, the kinetics of the development and decay of DTH were the same in treated and non-treated mice. Immunopotentiation was demonstrated by passive transfer: using the same number of spleen cells from treated or non-treated immune mice, higher DTH reactions were observed in normal recipients when they received spleen cells from P40-treated donor mice. Immunopotentiation seemed to act by increasing the number of committed lymphocytes and not by augmenting the number of accessory cells, since adoptive transfer of DTH by immune cells was not different in normal or P40-treated recipients.