A proteolytic system was found in all the species of dairy Propionibacterium. Two types of activities could act on [Cl-14] beta-casein or [Cl-14] alpha(s1)-casein. The first was the highest at the beginning of the exponential growth phase, tightly linked to the cells and hydrolysed preferably beta-casein. The second was released at the end of growth and acted similarly on both substrates tested. This activity was located in the cell membrane of these cheese-ripening bacteria. The enzyme could be gently extracted from the cell by incubation in Ca2+- or Mg2+-free buffer.