A fourth Na,K-ATPase alpha isoform, which was found to be abundant in testes, was proved to be a catalytical subunit of the enzyme. Recently, it has been shown that the alpha 4 isoform along with alpha 1 is expressed in the midpiece of the flagellum of mature rat sperm and the inhibition of alpha 4 with ouabain led to sperm immotility. In this study, sperm from 135 males with normal semen profile and 50 males with oligoasthenospermia were treated with 10(-5) and 10(-2) M ouabain solutions to inhibit alpha 4, and alpha 4 plus alpha 1 isoforms, respectively. In males with normal semen profile, sperm motility has been demonstrated to decrease with time to almost the same level with both ouabain solutions. In oligoasthenospermic males motility was also found almost completely lost. These observations showed us that the alpha 4 isoform may be held responsible for human sperm motility.