Fluorescence flow cytometry (FC) was employed to monitor the platelet surface in humans receiving intravenous infusions of Fab fragments of a chimaeric (murine/human) construct of monoclonal antibody 7E3, which binds to the fibrinogen receptor (glycoprotein IIb-IIIa) and inhibits platelet aggregation. Platelet surface-bound chimaeric 7E3-Fab (c7E3-Fab) was measured using a fluorescein-conjugated polyclonal anti-7E3 antibody and residual c7E3-Fab binding capacity was measured using fluorescein-conjugated c7E3-Fab. Turbidometrically measured platelet aggregation response to ADP was shown to be a linear function (r = 0.9) of the logarithm of residual free binding sites for c7E3-Fab. The distribution of c7E3-Fab in the platelet population was unimodal at all time points following the infusion, demonstrating in vivo transfer of antibody to newly released circulating platelets. Clearance of platelet surface-bound antibody followed an exponential model but all circulating platelets were bearing c7E3-Fab at time points well beyond the lifespan of the platelets exposed to c7E3-Fab during the infusion. Ex vivo and in vitro mixing experiments showed that c7E3-Fab transfer between platelets was possible and suggested that differences in the in vivo kinetics between monovalent and bivalent forms of monoclonal antibodies might be relevant in their therapeutic application.