Corynebacterium parvum injected i.p. 1--16 days prior to i.p. antigen inoculation virtually abolished both IgM and IgG primary responses to 1 X 10(8) SRBC. The suppression was significantly marked at antigen doses ranging from 1 X 10(6)--1 X 10(9) SRBC but not at 5 X 10(9) SRBC. As little as 56 microgram C. parvum caused a marked suppression of the response to 1 X 10(8) SRBC. In secondary responses C. parvum given either one day before priming with 1 X 10(8) SRBC or one day before secondary challenge caused a dramatic suppression of both IgM and IgG PFC responses. In contrast with i.p. injected C. parvum, i.v. injection of the vaccine enhanced immune responses to i.p. or i.v. injected SRBC. Similarly C. parvum injected i.p. prior to i.v. immunization resulted in an augmented anti-SRBC response. An enhancement of anti-SRBC response was also noted when C. parvum was injected i.p. on the day of i.p. immunization. The suppressed responses in C. parvum injected animals could be explained partly by the reduced splenic localization of the antigen.