This paper reports differences observed in the elemental content of fertile and infertile human spermatozoa used in an in vitro fertilization (IVF) program. "Fertile" and "infertile" were designated by the successful penetration or failure to penetrate an oocyte in vitro. We report morphological and morphometric differences which, together with elemental changes, may be causes of infertility in apparently normal spermatozoa. There were significant differences (P less than 0.05) in sodium and chlorine concentrations between fertile and infertile samples and there was more chlorine than could be accounted for as sodium chloride. Many spermatozoa showed particles adhering to tails, with a higher incidence of "contamination" in the infertile spermatozoa. There were significant differences in both shapes of heads and lengths of tails between fertile and infertile spermatozoa.