The stimulatory effects of poly A:U and poly I:C on antibody formation can be potentiated by the concurrent administration of theophylline or caffeine. This potentiation is dose-dependent and is absent, or may even lead to inhibited responses, when excessively high levels of theophylline or caffeine are employed. Potentiation is most effective by the subcutaneous route and stimulation expresses itself both in terms of rate of activation of antibody-forming cells as well as in circulating antibody titers. Theophylline, in the absence of poly A:U, stimulates antibody formation only when given subsequent to the time of immunization. These effects of theophylline and caffeine, both known stabilizers of cAMP levels, further support the conclusion that cAMP-mediated events are a critical component of the activation and regulation of immunocompetent cells.