Information concerning cell-wall associated proteinases of lactobacilli is limited. In Lactobacillus casei and Lactobacillus plantarum , presence of such proteinase is clearly shown. Differences between several strains were noticed. Higher cell-wall-associated proteinase activity can be measured in extracts obtained from milk-grown cells when compared to MRS-grown cells. No aminopeptidase, dipeptidase or carboxypeptidase activities were detected in the cell-wall-associated proteinase fraction. Isoelectric focusing of αs1-casein hydrolysates obtained by the action of this fraction from L. casei grown in milk revealed the presence of a major hydrolysis product and three minor degradation products with isoelectric points more acidic than αS1. Beta-casein was also degraded by the cell-wall extract with formation of one major product and several minor products with isoelectric points more acidic than β-casein. Two major hydrolysis products with isoelectric points higher than β-casein were also detected. Isoelectric focusing of αs1- and β-casein hydrolysates obtained by the action of the intracellular extracts of L. casei grown either in milk or in MRS broth shown identical patterns. As with L. casei , two strains of L. plantarum exhibited cell-wall proteinase activity. Milk-grown cells were more proteolytic than MRS-grown cells. Generally L. plantarum was significantly less proteolytic than L. casei .