The bacterium Variovorax paradoxus, grown in a minimal medium in which silk fibroin represents the sole source of carbon and nitrogen, produces an extracellular protease that hydrolyzes fibroin as well as casein and, to a smaller extent, collagen and albumin. The optimal pH for activity was found to be in the acid range (optimum pH 5.8-6.4) and the enzyme activity was stimulated by the addition of divalent cations, either manganese or magnesium. Gel permeation chromatography and SDS-PAGE provided evidence that the native enzyme is a monomer with a M r of ca. 21kDa.