To get a better understanding of the role of the previously reported fibrinogenolytic enzyme of Aspergillus fumigatus, we investigated the in vitro conditions of enzyme synthesis and attempted to characterize it. Modification of the nitrogen source did not influence the extracellular serine-proteinase profile, but resulted in important quantitative differences in the yields in batch cultures. The enzyme synthesis appeared to be an inducible phenomenon in A. fumigatus since it was initiated exclusively in the presence of proteins or protein hydrolysate. Free amino acids or inorganic nitrogen compounds could not promote significant enzyme production. Moreover, peptone at a concentration of 0.1% appeared to be the best inducer of enzyme synthesis. Conversely, modification of the carbon source did not affect fungal growth or enzyme synthesis. However, the production of chymotrypsin was highly sensitive to the carbohydrate level in the culture medium and, with peptone as nitrogen source, highest yields were obtained in the presence of 0.3 or 0.5% glucose. Culture filtrates of A. fumigatus CBS 113.26 grown with peptone or nitrate as nitrogen source were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Comparison of the protein patterns suggested for the proteinase a molecular mass of 33 kDa which was confirmed by chromatographic purification of the enzyme through (N alpha-CBZ)-D-phenylalanine agarose.