A simple and rapid histochemical technique is described for demonstration of delta 5,3 beta-hydroxysteroid dehydrogenase activity in cumulus cells from preovulatory follicles aspirated for in vitro fertilization (IVF) of corresponding oocytes. Histochemical activity of delta 5,3 beta-hydroxysteroid dehydrogenase was demonstrated in samples of cumulus obtained from 62 oocytes recovered from 24 women. Patients were treated with clomiphene citrate in combination with human menopausal gonadotropins and human chorionic gonadotropin injections. The cumulus was found to contain small and large cell types. Small cells possessed more delta 5,3 beta-hydroxysteroid dehydrogenase activity predominantly in the area near the oocyte. Cytoplasmic vacuolation has been noted in large, pale cells with moderate or low enzyme activity. The most active cells were predominant in cumulus from which oocytes were fertilized. Significant differences have been found between high and low delta 5,3 beta-hydroxysteroid dehydrogenase activity of cumulus cells from mature oocyte-corona-cumulus complexes leading to a successful fertilization and cleavage of oocytes and between groups with different histochemical activity when aspirated complexes were scored immature and the IVF of oocytes has failed.