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.