The protein inhibitor of cysteine proteases was isolated from an important zoophilic dermatophyte species Trichophyton mentagrophytes (T. mentagrophytes) and partially characterized. The isolation process involved affinity chromatography, followed by ion-exchange chromatography and reverse phase high performance liquid chromatography. The fungal inhibitor appears to exist in a high (24 kDa) and low (12 kDa) molecular mass form. It inhibits proteolytic activity of papain, cathepsins B and L but not of cathepsin H or trypsin. Results of immunoblotting procedures indicate that sera of T. mentagrophytes infected rabbits contain antibodies against higher molecular mass forms of the inhibitor. Since no sequence homology has been found between partial protein sequences of T. mentagrophytes inhibitor and other known cysteine protease inhibitors so far, we can speculate that this inhibitor has some structurally unique characteristics. The T. mentagrophytes inhibitor shares some biochemical similarities (molecular mass, high and low molecular mass forms, inhibitory profiles) with clitocypin from Clitocybe nebularis and macrocypins from Macrolepiota procera.