E. coli contains two distinct groups of proteolytic enzymes: one is serine proteases that hydrolyze globin and casein and the other is metalloproteases degrading insulin and glucagon. The levels of both activities were increased several-fold when the cells were grown to stationary phase in Luria broth (LB) but not at all when they were cultured in M9-glucose medium. `The raise in globin-hydrolyzing activity of serine proteases was completely prevented by the addition of glucose in LB, while insulin-degrading activity of metalloproteases remained increasing under the same growth condition. Starvation for carbon source of the cells grown in M9-glucose medium casused a marked increase in the activity of serine proteases but not of metalloproteases. Furthermore, the level of serine proteases did not increase in a mutant cell that lacks adenylate cyclase when the cell was grown in LB to stationary phase while the activity of metalloproteases was persistently raised in the same cell. Thus, the level of serine proteases in E. coli appears to be regulated by a mechanism similar to catabolite repression and that of metalloproteases by a change of intracellular pool of free amino acids.