Glutathione peroxidase (GPx) has a central role in the defense against oxidative damage: however, data on GPx activity in the buffalo semen is limited. To expand this knowledge GPx activity in the seminal plasma of buffalo-bulls was studied. Fifty-five semen samples were collected from 19 buffalo-bulls and evaluated before cryopreservation. Aspartate aminotransferase (AST) level was assayed in the seminal plasma before freezing and after freezing- thawing. The results revealed GPx activity (14.59 ± 0.50 U/ml) in seminal plasma that varied with age of the buffalo-bulls. Older age was associated with a decrease in GPx activity. Increasing GPx activity in the seminal plasma was accompanied by a significant de-crease in sperm motility of freshly collected semen. The rate of increase in post-thaw sperm motility in response to 5 mM pentoxifylline was high in samples with high GPx activity before freezing. Increased GPx activity before freezing resulted in high percentages of abnormal mid-piece and tails post-thawing. Positive correlations existed between AST levels in the seminal plas-ma (before freezing and after thawing) and frequency of-post-thaw sperm cell abnormalities.