Double antibody immunoassay of secretory aspartic proteinases from 2 strains of the yeast Candida albicans was performed in microtest plates with rabbit antibody and peroxidase conjugated rabbit antibody. The test detects approximately 0.1 ng of proteinase and is 2 orders of magnitude more sensitive than direct measurement of enzymic activity with hemoglobin as a substrate. The immunoassay was affected both by alkaline denaturation, which is a common feature of aspartic proteinases, and by the presence of the proteinase scavenger alpha-2-macroglobulin. Conditions are described that allow for comparison of proteinases from different strains of yeast and minimize the interference of macroglobulin.