Clinical disorders such as malignant diseases, infectious diseases or autoimmune diseases are associated with circulating immune complexes. These immune complexes can activate the complement system in the blood or interact with complement or Fc receptors on the surface of cells. Complement activation may cause cytolysis and the immune complex interaction with receptors may cause activation of cells. We have used flow cytometry and labelled chicken antibodies to study the in vitro effects of model immune complexes on platelets and show that such immune complexes activate platelets and deposit Clq, C4 and C5 on them. Either low levels or no C3 could be detected on the platelets by flow cytometry. The immune complexes also induced formation of microparticles from purified platelets. Flow cytometry might become a useful tool in estimation of risk of thrombosis or thrombocytopenia in patients with autoimmune disease. Chicken antibodies are superior to mammalian antibodies for the measurement of platelet bound plasma proteins as they do not induce complement activation or platelet activation.