Delayed-type hypersensitivity in mice to bovine serum albumin (BSA) which would be induced by a s.c. injection of BSA in Freund's complete adjuvant was interrupted by an i.v. injection of alum-precipitated BSA plus bacterial endotoxin (AP-BSA plus ET). In contrast, s.c. injection of BSA restrained the antibody response in the spleen of the mice receiving AP-BSA plus ET. Subcutaneous injection of either the adjuvant alone or an unrelated antigen in the adjuvant did not affect the anti-BSA antibody response in the spleen. The antibody response to other antigens in the spleen was not affected by the s.c. injection of BSA. The reduction of the antibody response in the spleen does not seem to be attributable to suppressor cells, since the suppressor cell activity against the antibody response of normal mice and that of irradiated recipients of primed spleen cells was not observed in the spleen cells of mice given s.c. immunization. The results indicated that the reduced antibody response in the spleen may be caused by the migration of some part of the antigen reactive cells or their precursors from the spleen to the s.c. region.