Diaphorase activities were measured in spermatozoa from 104 infertile men. The relationships between enzyme activity, sperm characteristics (density, motility, viability), and distribution of electrophoretic patterns were examined. There was a highly significant difference (p less than .001) between the sperm diaphorase activities in specimens with sperm concentration above 40 million per milliliter (0.685 +/- 0.5 mIU/10(6) sperm; n = 57) when compared with the diaphorase values of the oligospermic specimens with sperm density below 40 million per milliliter (1.53 +/- 1.5 mIU/10(6) sperm; n = 47). However, there was no evidence to support any relationship between sperm diaphorase activities, sperm motility, viability, and the distribution of three diaphorase electrophoretic patterns. This indicates that these parameters do not account for the differences between the diaphorase activity of spermatozoa of infertile men with sperm concentrations below and those above 40 million per milliliter.