Progesterone was transformed to its 11 alpha-hydroxy derivative (100% e.e) by the activated immobilized conidia of Aspergillus ochraceus TS. The immobilized preparation retained 79% of free conidial activity. The immobilized conidia, activated by nutrients, exhibited an increase in 11 alpha-hydroxylation, and it was free of the side product 6 beta, 11 alpha-dihydroxy progesterone. The half life and turnover of immobilized and activated immobilized conidia were 14 and 12 days and 187 and 416 mu moles of the product/g of conidia respectively. The pH and temperature profiles of the free conidia remained unaltered after immobilization and activation. Some germination of conidia inside the matrix owing to incubation with nutrients was detected by scanning electron microscopy.