The presence of Fc receptors, but not C3 receptors, on human and bovine spermatozoa was shown by sperm-induced rosette formation of antibody-treated ox erythrocytes and by immunofluorescence of aggregated human gammaglobulin (AHG) treated spermatozoa. Human and bovine spermatozoa bound AHG as well as circulating immune complexes (CIC) from human sera; monomeric IgG bound to these cells to a much lesser extent. Binding of AHG to the sperm surface also occurred when the AHG was suspended in either buffer or in serum that had been heated at 56 degrees C for 30 min. A bovine spermatozoa radioimmunoassay for the detection of immune complexes in human sera was developed and was shown to be comparable to the Raji cell radioimmunoassay (r = 0 . 81, P < 0 . 01) in the quantitation of soluble immune complexes from a variety of human sera. An enzyme-linked immunoassay was then developed, using bovine spermatozoa fixed to wells of microtitre plates, to simplify the immune complex detection assay. The technique provides an analysis that is rapid, does not require radioactive chemicals, expensive equipment, or cultured cells and yields values for immune complex determination both comparable and complementary to the Raji cell radioimmunoassay (r = 0 . 70, P < 0 . 01). The sperm assays detected soluble immune complexes in sera of vasectomized males, cancer patients and patients with autoimmune or dermatological diseases.