The metabolic nucleotides of washed human platelets can be subdivided into ethanol-insoluble (almost all as ADP) and ethanol-soluble pools. About 5 % of the total radioactively labelled metabolic nucleotides are found in the ethanol-insoluble pool. At least part, if not all, of the ethanol-insoluble ADP is bound to platelet actomyosin (thrombosthenin). The specific activity of the ADP in this pool is the same as that of the rest of the metabolic ADP and is not ‘superactive’. Addition of thrombin to washed human platelets previously incubated with [3H]-adenine did not alter the level of ethanol-insoluble ADP, as compared to similarly treated control platelets. Similarly, there were no changes in the radioactive ADP content of actomyosin extracted directly from washed human platelets after exposure to thrombin, as compared to controls. The metabolic ADP content of control platelet actomyosin might reflect the in vivo situation, but the possibility exists that mechanical damage during platelet isolation from plasma and subsequent handling could have caused the exchange. No evidence presented here indicates the involvement of platelet actomyosin in the platelet release reaction mechan