1. The distribution and properties of superoxide dismutase were examined in mammalian semen, and the enzyme was used to investigate the role of superoxides in metal-ion-catalysed lipid-peroxidation reactions in spermatozoa. 2. Superoxide dismutase activity was detected in seminal plasma and spermatozoa from all species studied, exceptionally high activity being found in donkey semen. The enzyme is easily solubilized from spermatozoa, as 85-90% of the total activity is released by cold shock, a relatively mild form of cellular damage. 3. Purification and characterization of the enzyme from supernatant fractions prepared from cold-shocked boar spermatozoa showed it to be cyanide-sensitive, to have a mol.wt. of 31 000, a pI of 5.9 and to contain 1.85 g-atoms of copper and 1.91 g-atoms of zinc per mol of protein. However, extensive sonication of spermatozoa released a small amount of a cyanide-insensitive enzyme, presumably a mangano superoxide dismutase, from the mitochondrial matrix. 4. The presence of superoxide dismutase in spermatozoa, either intracellularly or extracellularly, did not inhibit ascorbate/Fe2+-catalysed lipid-peroxidation reactions, suggesting that superoxides are not essential intermediates in this system.