In this study, we have used a platelet counting technique to measure platelet aggregation in whole blood from thrombocytopaenic patients who had platelet counts in the range 18 × 109/I-85 × 109/1. Aliquots of blood were incubated with platelet agonists in a shaking water bath at 37°C. Platelet aggregation was quantified by measuring the fall in single platelets counted in an Ultra-Flo 100 Whole Blood Platelet Counter. ADP (2 M), collagen (1 g/ml) and ristocetin (0.5-2 mg/ml) induced a fall in platelet count of 50%, 60% and 80% respectively. It is concluded that the present method can be used as part of the screening procedure to determine platelet function disorders when platelet counts are too low for aggregation studies with the more widely used photometric technique. A. R. Saniabadi, C. D. Forbes, J. Belch, Department of Medicine, Ninewells Hospital, Dundee, G. Lowe, Department of Medicine, Royal Infirmary, Glasgow, J. C. Barbenel, Bioengineering Unit, University of Strathclyde, Glasgow, UK.