Heparin solutions containing benzyl alcohol as a preservative increase intracellular cAMP concentrations in human lymphocytes by as much as 5 fold. Similar changes but of a lesser magnitude were also observed in human polymorphonuclear leukocytes and platelets and rabbit alveolar macrophages. Further analysis demonstrated that benzyl alcohol used as a preservative was responsible for this finding and that heparin per se had no effect on cAMP metabolism. The cAMP response to benzyl alcohol was a time- and dose-dependent process. In human peripheral blood lymphocytes, cAMP levels increased in a linear fashion over a benzyl alcohol concentration range from 0.015-0.30% (v/v). The increased cAMP concentrations in lymphocytes as measured by radioimmunoassay were not due to formation of a cross-reacting adenosine-alcohol adduct since the immunoreactive material produced was destroyed by phosphodiesterase and co-migrated with 3H-cAMP in thin layer chromatographic studies. Concentrations of benzyl alcohol which affect cAMP metabolism are commonly present in biologic studies employing heparin solutions. In view of these findings certain previously described effects of heparin may need to be reinterpreted and in the future heparin solutions containing benzyl alcohol should not be used when studying biologic processes known to be influenced by alterations in cAMP metabolism.