Mutant "acatalasemicm" mice have an unstable catalase in hepatic and renal peroxisomnes that is readily degraded, apparently into peroxidase subunits. The hypothesis that an in situ cataloperoxidase would affect serum lipids was tested in these mutants and serum triglycerides and cholesterol were found to be significantly lower than in the wild strain. This finding is in accordance with reports that a hypolipidemic response to the injection of peroxidase subunits of hepatic catalase occurs in humans and rabbits.