It has been reported that drugs which modulate the intracellular levels of 3' 5' cyclic adenosinemonophosphate (cAMP) interfere with the cellular function of the immune system. In this work, papaverine, a drug effective in elevating cAMP levels, was observed to increase the T4+/T8+ ratio in vivo. Furthermore, peripheral blood mononuclear cells (PBMC) from young healthy volunteers who received intravenous papaverine (3 mg/kg i.v. over 15 min) showed a decreased response in allogenic mixed lymphocyte reaction (MLR) but a normal behaviour in a polyclonal mitogen assay and in autologous MLR (AMLR). These findings suggest that papaverine administration could influence the immune system functions.