Androgens (androstenedione and testosterone) belong to the most important compounds in human steroidogenesis. The 17 beta-hydroxysteroid dehydrogenase responsible for interconversion of the oxygenic group on C-17 of androgens ring is involved in steroid hormone synthesis. The fission yeast Schizosaccharomyces pombe 972 h- was found to contain constitutive 17 beta-hydroxysteroid dehydrogenase that was able to reduce androstenedione to testosterone and oxidize testosterone to androstenedione. The reductive pathway was found to be predominant while the oxidative one was carried out with much lower activity. Exogenous androstenedione, contrary to testosterone, inhibited S. pombe growth and stimulated the formation of aberrant swollen cells with slighter cell wall sensitivity to the action of the lytic enzyme Novozym. It is postulated that the 17 beta-hydroxysteroid dehydrogenase prevents the deleterious effects of androstenedione on the morphology and growth of the yeast's cells by androstedione reduction to testosterone.