The presence of the secretory aspartate (acid) proteinase in the vaginal fluid of candidal vaginitis patients and controls was studied by ELISA and immunoblot (Western blot). In addition, a proteinase-deficient mutant strain of Candida albicans (IR24) was compared with the wild-type parent strain (10261) for ability to infect the vagina of pseudoestrus rats under estradiol treatment. Among the 67 women examined, proteinase was detected only in 22 harboring C. albicans (range, 42-233 ng/ml of vaginal fluid), at concentrations significantly higher in the 14 vaginitis patients than in the 8 asymptomatic fungal carriers. Western blots confirmed the presence of only one protein band of approximately 43 kDa, corresponding to that of the purified proteinase, in the ELISA-positive vaginal fluids. Experimental vaginal infection was significantly more extensive and persistent in rats infected with the proteinase-producer strain than in those challenged with the proteinase-deficient mutant, and the enzyme was detected in the vaginas of the former but not of the latter animals. Both strains 10261 and IR24 developed hyphal forms to a roughly similar extent during infection, and both showed a comparable adherence in vitro to vaginal and buccal epithelial cells. The clinical and experimental evidence support a role for secretory proteinase as a virulence factor in the pathogenesis of candidal vaginitis.