An immunoperoxdase technique in electron microscopy was used to investigate the ultrastructural site of keratinolytic proteinase (KPase) of M crosporum cans in sections of skin from guinea pigs infected with the same organism. Ultrastructurally, the KPase was present only in the cell walls of the invading dermatophytes as a continuous deposition of the electron-dense reaction product on the inner and outer aspects of the cell wall of the fungal hyphae without deposition in the keratin surrounding the invading hyphae. Our results suggest that the KPase may not play an absolute role in the invasion of dermatophytes into keratinized tissue in vivo.