Both the complement system and platelet-leukocyte aggregates are involved in chronic and acute stages of atherosclerosis. Properdin, a positive regulator of the complement system, is secreted by leukocytes and endothelial cells. In the present study, the role of properdin in the formation of platelet-leukocyte aggregates was investigated. Incubation of human whole blood with properdin (25-200 microg/ml) resulted in a dose-dependent formation of platelet-leukocyte aggregates, with an increase of up to 2.2-fold compared to controls (p < 0.05), as analysed by flow cytometry. In addition, properdin significantly amplified ADP-induced aggregation of platelets with leukocytes by 53% (p < 0.05), while it had no effect on ADP-induced aggregation of platelets alone. Consistent with these results, properdin did not activate platelets as shown by the expression of activated GPIIb/IIIa (PAC-1 epitope) and P-selectin (CD62P) on the platelet surface. However, properdin significantly induced expression of CD11b (MAC-1) on leukocytes by 12-fold (p < 0.05) as a measure of leukocyte activation. In conclusion, the complement system component properdin induces the formation of platelet-leukocyte aggregates via leukocyte activation. The data establish a link between the complement system and platelet-leukocyte aggregates with potential significance in atherosclerotic vascular disease.