Impedance aggregometry allows the measurement of platelet responses in whole blood as well as in PRP. The variability of haematocrit values encountered when applying this technique to haemodialysis patients prompted an investigation of the effects of red cells on platelet aggregation in whole blood. Collagen induced aggregation was measured in both PRP and whole blood from haemodialysis patients and healthy controls. Platelets from haemodialysis patients were less aggregable than those from the controls when tested in PRP, but more aggregable when tested in whole blood. Blood samples with a range of haematocrit values were prepared by mixing PRP and autologous red cells, and used to study the effect of haematocrit on platelet aggregation. In blood from control subjects aggregation rate was reduced by rising haematocrit but no reduction of maximum aggregation occurred until haematocrit exceeded 40%. In contrast uraemic platelets showed increased responses in the presence of red cells. In a limited cross over study no significant difference was found in the effect on platelet aggregation of washed erythrocytes from uraemic and non-uraemic donors. It is concluded that red cell presence influences platelet aggregation by complex mechanisms during impedance aggregometry and that this effect must be considered when interpreting results.