The presence of beta 2 adrenoceptors on the human platelet is well established although the coupling of this receptor to adenylate cyclase is disputed. In this study of highly purified platelets isoprenaline did not stimulate whole platelet cyclic AMP alone or in the presence of forskolin. Minimal contamination of the purified platelets (approximately 0.05%) with mononuclear white blood cells may lead to the spurious appearance of isoprenaline stimulated cAMP. These results confirm our previous findings of lack of coupling of the human platelet beta 2 adrenoceptor to adenylate cyclase and indicate the importance of purity of preparation for platelet biochemical studies.