The variation in the relative rate of catalase synthesis by Bact. lactis aerogenes has been studied by measuring the enzyme activity of samples of cells taken at intervals from growing cultures, under different conditions. In the normal growth cycle in a glucose-ammonium sulphate medium the rate of catalase synthesis is small during the early part of the logarithmic phase, but the content per cell increases rapidly as the pH of the medium falls. The increased activity is not a direct result of the acidity itself, but may be attributed to modification of the cell metabolism under acid conditions so that more catalase is required. Continued growth under anaerobic conditions tends to reduce catalase activity, although the characteristic variation of the relative rate of synthesis through the growth cycle is maintained. The variation of the rate of formation of the enzyme in cells growing in a succinate medium is similar to that in glucose, although synthesis is now appreciable even in the early stages of growth.