Whole cells of Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397 were able to hydrolyse alpha- and beta-caseins. Irrespective of the growth medium used, milk or De Man-Rogosa-Sharpe (MRS) broth, identical patterns of alpha- and beta-casein hydrolytic products, respectively, were visualized by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A soluble proteinase present in cell-wall extracts was active on caseins and displayed the same hydrolytic patterns as whole cells. It was purified from cell-wall extract to homogeneity by ultrafiltration and ion exchange chromatography. The enzyme is a monomer with a molecular mass of 170 kDa, an optimum temperature of 42 degrees C and an optimum pH of 5.5. It was strongly activated by dithiothreitol and partially inhibited by E-64. These properties indicate that cysteine residues play an important role in the enzyme mechanism. The purified proteinase was not able to hydrolyse di- or tripeptides.