The intracellular proteinase pumAi in Ustilago maydis has been associated with yeast-mycelium dimorphic transition. The proteinase was purified from a cell-free extract by ammonium sulfate fractionation and chromatographic steps including hydrophobic interactions on a Phenyl Superose column, ion exchange on a Mono Q column, and gel filtration on Superose 12 columns. The enzyme has a mass of 35.3-36.6 kDa, a pH and temperature optimum of 4.0 and 40 degrees C, respectively, and a pI of 5.5. The enzyme degraded hemoglobin, gelatin, albumin, and casein, but not collagen, and the enzymatic activity was strongly inhibited by pepstatin A, an aspartyl proteinase-specific inhibitor. The biochemical characteristics of pumAi are similar to other fungal intracellular aspartyl proteinases, however, this is the first biochemical characterization of a basidiomycete proteinase probably associated with dimorphic yeast-mycelium transition.