During the log-phase growth of Proteus mirabilis the specific activity of catalase decreases, while at the beginning of or during the stationary phase an increase takes place which is abolished by inhibitors of nucleic acid or protein synthesis. Glucose in the culture medium has no appreciable effect on the level of enzyme synthesis nor does the passage of bacteria to anaerobiosis bring any noticeable change. Successive additions of hydrogen peroxide up to weak final concentrations (0.2--0.5 mM) stimulate catalase synthesis. Determination of the enzyme in vivo reveals but a weak proportion of the total catalase which can only be titrated after the breakdown of cells. The titrable enzyme in vivo represents, as an order of magnitude, the activity found associated with the cell wall, in an easily released form after the mechanical separation of the inner and outer membranes. Thus, bacteria can act upon exogenous peroxide only through a peripheral catalase while they possess in a masked form an important reserve of cytoplasmic enzyme.