Rapid and relevant evaluation of platelet function is often clinically important. By means of fluorescent labelled chicken antibodies (which do not bind to Fc-receptors) against fibrinogen and von Willebrand factor and flow cytometry, we have determined the time course of ligand association to platelets after stimulation with adenosine 5'-diphosphate and ristocetin respectively. The expression of guanosine 5'-phosphate (GMP)-140 was also measured. We have applied this technique to evaluate platelet function during platelet storage and cardiopulmonary bypass. There was a significant reduction of the binding of fibrinogen and von Willebrand factor and significantly increased expression of GMP-140 after 9 days of storage. Changes in metabolic variables such as lactate accumulation, glucose consumption and decrease in pH confirm that the functional impairment is due to a large extent to a deteriorated platelet metabolism. No significant differences were found between samples taken before and during cardiopulmonary bypass, but there was a tendency towards increased ligand binding as well as increased expression of GMP-140 at the end of cardiopulmonary bypass. The flow cytometric technique that is described may be useful for evaluation of platelet function and platelet activation in vivo.