Arginase(Arg) activity in seminal plasma and sperm cells from infertile men and healthy fertile donors was measured. There were no statistically meaningful differences in seminal plasma Arg activity between the two groups whereas sperm cells from oligospermic infertile men had a higher Arg activity compared with the controls. Some important correlations were established between sperm count and Arg activity (negative values) and sperm motility and Arg activity (positive values) in both sperm cells and plasma samples from infertile men. Results suggest that the arginine-nitric oxide pathway within sperm cells from oligospermic infertile men is disturbed by enhanced Arg activity. We think that this may play a part in sperm dysfunction and male infertility.