The functional integrity of the membrane of sperm from semen samples collected from 10 fertile men (Group A) and from 50 infertile men (Group B) was assessed by studying the swelling reaction of sperm when suspended in a medium of distilled water ('Water-test'). The results were correlated with routine semen-analysis and with the results of Eosin-Y staining. The mean values for the 'Water-test' were significantly different (P less than 0.01) between Groups A and B (84.8 +/- 3.5 versus 70.9 +/- 13.3, respectively). No significant correlations were observed in either Group A or B, between values for the 'Water-test' and values for the sperm count, the percentage of sperm with normal morphology and the percentage of motile sperm. There was a good correlation in both Group A (r = 0.86, P less than 0.01) and Group B (r = 0.92, P less than 0.01) between values for the 'Water-test' and those for the Eosin-Y test. These results indicate that the 'Water-test' is a simple and reliable test for evaluating sperm membrane integrity.