Nitraquazone (TVX 2706) is an phosphodiesterase inhibitor with antiinflammatory action. It strongly enhances the increase in intracellular levels of cyclic AMP caused by appropriate effectors in all systems tested so far. EC50 values are in the submicromolar range. The effect is apparently neither due to an increased responsiveness of the hybrid cells for an effector like prostaglandin E1 nor to an increased activity of adenylate cyclase, but to an inhibition of both low and high affinity cyclic AMP phosphodiesterases. Half-maximal inhibition of enzyme activity is obtained at 10 uM TVX 2706. TVX 2706 does not interfere with the calmodulin activation of phosphodiesterase. Nitraquazone affected cytokine production by PHA-stimulated human blood cells. Nitraquazone was active in reducing the production of IL-5 (IC50 = 0.8 uM), while its potency against IL-2, GM-CSF and IFN-gamma was 3-6 times lower.