Clinical isolates of C. albicans (75 strains) and other yeasts (20 strains) were evaluated for their ability to produce a carboxyl acid proteinase in an effort to assess its potential role as a virulence factor. Yeasts were categorized as to the infectious process present in the patient: (1) isolates from patients with invasive disease, (2) isolates from patients with possible invasive disease, (3) isolates from superficially infected patients and (4) isolates from noninfected, colonized patients. Yeasts were grown for 7 days in medium containing bovine serum albumin (BSA) as the sole nitrogen source. The amount of extracellular proteinase was measured at pH 3.2, using BSA as substrate. The majority (97%) of C. albicans isolates produced a detectable proteinase. Some non-C. albicans isolates produced proteinase; however, the amount of activity was generally less than for C. albicans. No correlation was found between the amount of proteolytic activity and the degree of invasiveness of the strains.