Exp. showed the peroxidase of pig''s spleen to be partly water-soluble and partly associated with insoluble material. Since various attempts to remove catalase without affecting peroxidase failed, a method for estimating peroxidase in the presence of catalase is described. Peroxidase activity was highest in spleen and lung; liver and kidney contained a little (which was removed by perfusion with Ringer solution, and hence was attributed to Hb in blood corpuscles) ; in most other tissues it was negligible. There was possibly some true peroxidase in spleen and lung. The peroxidase activity of various hematin and Hb derivatives was given.