Abstract The activity of catalase in M. lysodeikticus and its protoplasts and lysates has been investigated over the pH range 4–9. The results suggest that the intracellular catalase is protected from change in the pH of the external environment by a barrier impermeable to the H + ion. The effect of a series of lytic agents upon the catalase activity indicates that this barrier is the plasma membrane of the organism. The changes with pH of the activation energy of the overall catalase-H 2 O 2 reaction in the three bacterial preparations provide support for this interpretation. It is suggested that the intracellular bacterial catalase is operating at an effective pH of 4.6−4.8.