Seminal plasma from ejaculates of 10 healthy, fertile volunteers and 63 infertile males was analysed for superoxide dismutase (SOD) and xanthine oxidase (XO) activities using a chemiluminometer. There was not statistically significant difference in the activity of either enzyme between control and infertile populations (113 +/- 74 IU/ml for SOD and 1.17 +/- 0.52 IU/ml for XO) in samples from normozoospermic ejaculates. Sperm progressive motility was positively correlated with SOD activity in seminal plasma of corresponding ejaculates (P < 0.05) and negatively with XO activity (P < 0.001). An 'oxido-sensitive' index was defined as the SOD/XO ratio and was found to be inversely related to sperm progressive motility samples (P < 0.01). Analysing this index among all tested samples of semen including those with pathological spermiograms, as well as normospermic (N) samples we found statistically significant (elevated) differences in oligoasthenoteratospermia (OAT) in comparison with N (P <0.05); OAT samples were also significantly different from oligospermic (O) and oligoteratospermic (OT) samples (P < 0.05). This suggests that the 'oxido-sensitive' index of seminal plasma may be a simple diagnostic factor, useful in the determination of male infertility.