The catalase in washed suspensions of Bacterium lactis aerogenes possesses the same characteristics as the cell-free prepns. of the enzyme obtained from other sources. The catalase activity is maximal for cells which have been grown in the presence of Fe++ at 2.5 X 10-6 g. ions/I. Cells adapted to sulfanilamide show an increase, and those which have been grown in the presence of proflavine show a decrease, in catalase activity.