By electron microscopic studies it could be demonstrated that malformed outer dense fibres are a major cause of structural, flagellar disorder in human spermatozoa. Biochemical investigations of isolated outer dense fibres revealed that these flagellar substructures consist of three major polypeptides and do not contain phosphoproteins. They seem to have passive-elastic properties with respect to sperm motility. Further studies are required to elucidate the cause of the disturbed development of outer dense fibres, which may be due to inflammatory, toxic, or genetic influences.