Dimethyl sulfoxide (DMSO) is used as a cryoprotective agent when platelets are frozen. We examined the effect of DMSO (0.1 to 10%) on platelet aggregation, release, and prostaglandin synthesis (as indicated by malondialdehyde formation) in response to thrombin, collagen, arachidonic acid and calcium ionophore. Inhibition was observed at the lowest levels of DMSO, varied with the type of stimulus, and was reversed by washing the platelets. Inhibition of aggregation, release, and malondialdehyde formation were dose-dependent with thrombin or collagen. DMSO did not inhibit malondialdehyde formation stimulated by arachidonic acid, nor did it consistently inhibit any function stimulated by calcium ionophore. When platelets were stored as platelet-rich plasma at 20 to 24 degrees C for 48 hours, with and without 5 percent DMSO, and subsequently washed, the platelets stored with DMSO were more reactive in vitro. These results indicate that platelet function inhibition by DMSO not only is reversible, but protects the platelets during storage. The factor limiting the use of DMSO in platelet storage is potential systemic toxicity, not its effects on platelets.