The effects of in vivo acetylsalicylic acid (ASA) treatment on the fibrinolytic activity of peripheral blood granulocytes have been studied in a group of healthy volunteers. Following a 4-day treatment with aspirin at a dose of 1 g/day, both the plasminogen-dependent and the nonspecific fibrinolytic activity were significantly decreased. On the contrary, no significant change in the granulocyte fibrinolytic activity was found after treatment with 250 mg ASA/day. It is suggested that the cellular release mechanism of such lytic enzymes rather than their synthesis could have been affected by the drug.