Binding of fibrinogen to platelet glycoprotein (GP) IIb/IIIa receptors is essential for normal platelet aggregation. We investigated whether inhibition of GP IIb/IIIa receptors with arginine-glycine-aspartate-O-methyltyrosine amide (RGDY), an analog of the receptor recognition sequence found in fibrinogen, is associated with platelet deaggregation. Platelets from five healthy human subjects that were maximally aggregated by addition of adenosine diphosphate to platelet-rich plasma were deaggregated in a dose-dependent manner by subsequent addition of RGDY (86.5 +/- 6.2%, 68.2 +/- 4.3%, 44.9 +/- 6.4%, and 31.6 +/- 4.1% for RGDY concentrations of 400, 200, 133, and 67 microM, respectively vs. 7.8 +/- 2.9% for saline control samples, p less than 0.0001). The extent of deaggregation was decreased as the time of addition of RGDY (400 microM) after maximal aggregation increased (85.6 +/- 6.7%, 58.5 +/- 12.6%, 47.1 +/- 2.7%, and 37.1 +/- 4.2% for 0, 1, 3, and 5 minute intervals, respectively, vs. 8.3 +/- 3.1% in control samples, n = 4, p less than 0.0001) consistent with the occurrence of irreversible aggregation. Thus, RGDY can rapidly and extensively deaggregate human platelets under certain conditions which may enhance its antithrombotic efficacy.