Cholera enterotoxin (CE) is a potent adjuvant when administered simultaneously with sheep red blood cells (SRBC) and an immunosuppressant when given 12 hr before or after the antigen. Antigen clearance is not modified, but splenic adenyl cyclase activity is significantly enhanced reaching peak levels within 12 hr. Theophylline enhances the modest adjuvant effect of 0.01 µg of CE and inhibits the pronounced adjuvant effect of 0.05 µg. The relationship between adenyl cyclase stimulation and modulation of the immune response is discussed.