Treatment of membranes from guinea-pig peritoneal eosinophils with deoxycholate and NaCl solubilized greater than 95% of the particulate cyclic AMP-specific phosphodiesterase (PDE IV). Solubilized PDE IV was at least 10 times more potently inhibited by selective PDE IV inhibitors (e.g. rolipram, denbufylline) than bound enzyme. Vanadate/glutathione complex (V/GSH) activated membrane-bound PDE IV and also increased potencies of these same inhibitors by at least 10-fold. Neither solubilization nor V/GSH markedly influenced the inhibitory activities of non-selective inhibitors (e.g. trequinsin, dipyridamole). Inhibitor effects on solubilized PDE IV and cyclic AMP accumulation in intact cells were strongly correlated. These results suggest a biologically important site on eosinophil PDE IV which is concealed or partially concealed in freshly prepared membranes and is exposed by solubilization or V/GSH.