In 30 patients we investigated the expression of activated GPIIb/IIIa-complex as an indicator ofin vivoplatelet activation before and after coronary angiography/angioplasty. Patients were divided into three groups according to aspirin treatment: group I: patients having a diagnostic coronary angiography without aspirin (n= 6); group II: patients having a diagnostic coronary angiography with low dose aspirin therapy (n= II); group III: patients having a coronary angioplasty with low dose aspirin therapy plus aspirin i.v. (n= 13). Platelets were identified in a flow cytometer by their characteristic light scatter profile and binding of anti-GP Ib, and activated platelets by an antibody to the activated GP IIb/IIIa. Cardiac catheterization lead to an increase of the mean anti-GP IIb/IIIa-fluorescence of the platelets and of the percentage of platelets with an anti-IIb/IIIa-FL exceeding a fixed threshold value (subpopulation). While aspirin substantially inhibited the increase induced by diagnostic angiography, the increase in the angioplasty group was the greatest of all groups despite aspirin.