The effects of metallic copper on human spermatozoa are described. Incubation with the metal caused a fall on the percentage of motile sperm, which was directly related to the surface area of copper employed and to the copper content of whole semen. These changes were accompanied by a decrease in semen zinc levels and an uptake of copper by individual sperm cells, as determined by X-ray microanalysis. Low concentrations of ionic copper caused a less marked fall in sperm motility, although the metal was generally more toxic than zinc or cadmium ions. All three metals were accumulated by spermatozoa on incubation, with cadmium causing a decrease in sperm nuclear zinc concentrations.