AbstractWhole cells ofPseudomonas testosteroni, induced to synthesize steroid‐transforming enzymes beforehand, have been immobilized by entrapment in polyacrylamide gel. The immobilized cells have then been used to catalyze the continuous Δ1‐dehydrogenation of Reichstein's substance S under various conditions in the presence of phenazine methosulfate (PMS), an electron acceptor for the cell‐free Δ1‐dehydrogenase. The presence of PMS substantially increases the rate of reaction when fed with the steroid substrate to a continuous stirred tank reactor containing the immobilized cells. The operational half‐life of the Δ1‐dehydrogenase activity of the cells, about 103 hr under the best operating conditions, is essentially unaffected by the presence of PMS. Though the acceleration of the reaction may be due to PMS‐mediated passage of electrons from some component in the electron transport chain to molecular oxygen, the lack of a similar effect with methylene blue is consistent with the conclusion that PMS functions directly as the electron acceptor for the