Intravenous injection of 2,000 rad of irradiated allogeneic cells can suppress the development of antigraft delayed-type hypersensitivity (DTH) to major and minor histocompatibility (H) antigens which normally arises after s.c. immunization. Secondary type DTH responses to minor H antigens were also largely suppressed by an i.v. injection of irradiated allogeneic cells 1 week preceding the s.c. priming injection. The extent of suppression of primary DTH to allogeneic H-2-incompatible cells depended on the dose of i.v. injected irradiated cells. After a dose of 1 x 10(7) irradiated spleen cells i.v., the suppression persisted for at least 40 days. Intravenous injection of cells incompatible for minor H antigens could not suppress the DTH to H-2 alloantigens and vice versa. Suppression of DTH to H-2 alloantigens was haplotype specific. Proliferation studies indicated that the immunosuppressed mice do not respond upon s.c. immunization with an increased proliferative activity in the draining lymph nodes, in contrast to nonsuppressed mice. The data suggest that i.v. preimmunization with allogeneic cells induces specific suppression of antigraft immunity acting at the induction stage of the immune response.