Detection and ultrastructural localization of aspartyl proteinase (Sap) in Candida albicans experimentally infecting rat vagina were studied. Two Sap-positive (Sap+) and one Sap-negative (Sap-) strains of the fungus, endowed with high and low experimental vaginopathic potential, respectively, were used. Both Sap+ strains produced consistent Sap levels in the rat vagina, while the Sap- strain did not produce any measurable Sap. Electron microscopy of thin sections of chemically-fixed vaginal scrapings showed clear evidence of hyphae of proteolitic strains of C. albicans invading the keratinized epithelial cell layer of the vagina. The fungal cells exhibited a pronounced fibrillar layer on the cell wall with a marked intermixing of fungal and vaginal materials especially pronounced at the hyphal tip. Post-embedding immunogold techniques with the use of anti-Sap polyclonal and the specifically generated monoclonal antibody GF1 showed that Sap was essentially localized in the cell wall of C. albicans early during infection, in a cytological pattern mirroring Sap localization in C. albicans cells grown in Sap-inductive media in vitro. In summary, the data offer a new biochemical and ultrastructural evidence that Sap is actively secreted during experimental rat vaginitis by C. albicans. Cell wall localization of Sap is probably inherent to this active secretion process.