SummaryThe relationship between superoxide anion production and the activities of sperm specific NADH‐diaphorase and isozyme LDH‐C4has been investigated in 25 normozoospermic and 17 oligozoospermic semen samples. The samples were separated into high and low density sperm populations on discontinuous, two step (40%:80%) Percoll gradients prior to analysis. Superoxide anion generation was significantly higher (P<0.001) and diaphorase activity significantly increased (P<0.01) in both low and high density sperm fractions from oligozoospermic patients compared to the values found in normozoospermic men. A significant positive correlation (P<0.01) between diaphorase activity and superoxide anion production was established in the oligozoospermic samples of low density sperm fraction. A tendency of statistically insignificant increase in LDH‐C4isozyme activity was observed in the oligozoospermic samples. The activity of this isozyme appears to have no association with superoxide anion generation or diaphorase activity. These findings seem to suggest that excessive superoxide anion production along with increased diaphorase activity point to biochemical abnormalities which may affect the functional status of sperms. Hence, determination of these parameters may provide useful data in male infertility of unknown eti