Abstract.Following induction of the microsomal liver enzymes, lower total prednisolone concentrations in plasma and an altered pattern of the prednisolone metabolites generated were observed. The purpose of the present study was to establish whether—as a consequence of an enhanced clearance—lower unbound concentrations of prednisolone in plasma are detectable after enzyme induction and to determine if the altered prednisolone metabolism modulates the biological effect.Before and during phenytoin dosage in ten volunteers, the mean (±SD) total body clearance (ml min‐1kg‐1) of both total and unbound prednisolone increased from 2·74±0·47 to 3·94±0·66 (P<0·001) and from 10·76±2·68 to 16·00±3·17 (P<0·001), respectively. These increments were due to increased non‐renal clearances. The immunosuppressive activity as a function of time in plasma determined as the percent inhibition of the mixed lymphocyte reaction decreased by one third after phenytoin dosing, while theEC50values of unbound prednisolone were unaffected. Thus enzyme induction decreases both the unbound prednisolone concentration