Cholera exoenterotoxin (CT) from Vibrio bacilli enhances antibody responses when administered together with an antigen such as sheep erythrocytes but suppresses immune responses when given several days before antigen. Although CT is known to stimulate adenylate cyclase activity and alters cyclic AMP levels in lymphoid as well as nonlymphoid tissues, the mechanisms of immunologic modulation by the toxin are unknown. In the present study the in vitro hemolytic antibody plaque response to sheep red blood cells was found to be markedly enhanced when splenocytes were obtained from mice treated with CT on the day of culture initiation or 1 day earlier, similar to the enhancement of antibody formation in vivo when mice were given CT and antigen at the same time. Spleen cells from mice treated 2 to 3 days earlier with CT, however, showed marked impairment of in vitro responsiveness to sheep erythrocytes, similar to the immunosuppression observed in vivo in toxin-pretreated mice. In contrast, CT induced only an enhanced antibody response when added to normal spleen cells in vitro, either on the day of culture initiation and immunization or 2 days earlier, suggesting that suppression of immunity induced by CT pretreatment in vivo is due to an indirect effect on cells other than immunocytes. This was substantiated by experiments with adrenalectomized mice since only enhancement and not suppression of antibody responses occurred. Impairment of spleen cell responsiveness from CT-pretreated mice could be partially restored by addition of PE cells and fully restored by “educated” T cells derived from recipient mice given thymocytes and SRBC. Thus, CT suppression of antibody formation may be due to an indirect effect on specific cell populations, whereas immune enhancement was probably due to a direct adjuvant effect of the toxin on immunocytes or their precursor.