The effect of thalidomide on a rat limb bud system has been studied in vitro. Thalidomide and adult monkey liver supernatants activated with cofactors for the mixed function monooxygenase system inhibited growth. Similar preparations from the rat did not inhibit growth. Planimetric measures of the surface area of limb buds were found to correlate closely with their protein contents. The ability to compare metabolizing systems from different mammals may be useful in extrapolating teratologic data between different species.