We have applied flow cytometry to the detection of activated platelets in patients with coronary heart disease. Paraformaldehyde-fixed platelets were incubated with one of the following monoclonal antibodies (MAbs): Bx-1 (anti-GP Ib), AP-2 (anti-GP IIb-IIIa complex), VH10 (anti-GMP-140, a glycoprotein of the alpha-granule membrane), or PAC-1 (directed against an activation-dependent determinant on GP IIb-IIIa complexes). Bound antibody was quantitated after the addition of FITC-conjugated anti-immunoglobulin. This report highlights studies on 16 unstable angina patients undergoing transluminal angioplasty. Blood samples were taken at different periods before and after the angioplasty. Levels of activated platelets were variable, remaining in the 2-4% range of control donors for some, but increasing to 10-30% post-angioplasty for others (despite all patients receiving heparin and aspirin). Maximum numbers of activated platelets were detected at 24 or 48 h. Nonetheless, the amount of antibody bound to individual platelets rarely reached the levels seen when control platelets were stimulated with thrombin in vitro. Results with VH10 and PAC-1 often, but not always, correlated suggesting different pathways of platelet activation.