Ascorbic acid at concentrations between 0.57 and 5.7 mM in aerated medium caused an eight fold increase in catalase activity in Escherchia coli. The hydrogen peroxide concentrations resulting from ascorbate oxidation were between 20 and 120 μM and hydrogen peroxide by itself caused a similar increase in catalase levels in both aerobic and anaerobic media. Three catalase activity bands visualized on polyacrylamide gels were increased. Chloramphenicol which inhibits protein synthesis, anaerobic medium and EDTA, which prevent ascorbate oxidation, and exogenous catalase, which removes hydrogen peroxide from the medium, all prevented the increase in catalase in response to ascorbate. Superoxide dismutase activity was not affected by ascorbate. © 1981.