11 beta-Hydroxysteroid dehydrogenase (HSD) activity was measured in freshly frozen granulosa cells isolated from follicles of twenty-one infertility patients undergoing in vitro fertilization-embryo transfer (IVF-ET). A total of 213 follicles were analyzed for 11 beta-HSD activity. Both nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP) dependent 11 beta-dehydrogenase activities were measured in granulosa cells. The activity in reductive direction (11-oxoreductase activity) was not measurable either with NADH or NADPH as cofactors. NAD- and NADP-dependent dehydrogenase activities are in comparable levels at 100 nmol/l and 1 mumol/l corticosteroid concentrations. For comparing enzyme activities of individual follicles, significant enzyme activity was considered to be a level of > 2 nmol/l/min/mg. 41.3% of the follicles demonstrated enzyme activity, 58.7% did not. The mean 11 beta-dehydrogenase (11 beta-DH) activity was calculated for each patient. Those patients with levels > 2 nmol/l/min/mg were considered enzyme positive; those with mean levels < 2 nmol/l/min/mg were considered negative. No significant association was noted between follicle size, oocyte maturity or fertilization rates and 11 beta-DH activity. This study noted the presence of 11 beta-DH activity in granulosa cells, however, no association with oocyte maturity and fertilization was found.