This study is based upon an analysis of 27 severely oligozoospermic men, in whom defects in sperm motility and morphologic defects were also evident, and 35 control subjects of proven fertility. Semen samples recovered from these men were assessed by conventional criteria, time-exposure photomicrography, and the zona-free hamster egg penetration test. Evidence of severely defective sperm function was obtained for the oligozoospermic group, which gave a mean +/- standard error (SE) fertilization rate of 2.8% +/- 1.5, compared with 44.0% +/- 3.4 for the normal fertile control subjects. Multivariate discriminant analysis was used for selection of those parameters of semen quality that could most accurately identify the 70% of oligozoospermic samples that failed to exhibit any fertilizing potential in the hamster egg assay. This analysis correctly identified 89% of such samples, on the basis of seven discriminating variables, of which the most significant were percentage of motile sperm, the log of sperm density, and a log transformation of the concentration of progressive spermatozoa.