Toxicity of heme pigments, in addition to that previously reported has been demonstrated as follows: By depression of oxygen uptake by bilirubin bathing relatively intact cells of yeast and diaphragm; by hematin bathing yeast, diaphragm and minced brain, and by mesobilirubin bathing mouse diaphragm. Oxidation cancels the toxicity of bilirubin. Cytochrome C cancels the toxicity of bilirubin and hematin. Mesobilirubin was not tested with cytochrome C. By immobilization of Tetrahymena pyriformis by bilirubin at a concentration as low as 15 mg./100 ml. Oxidation of the bilirubin or the addition of cytochrome C cancels the action of the bilirubin. Hematin does not immobilize the animals. Removal of the bilirubin restores the protozoa to full activity. Survival of newborn rats following intraperitoneal injection with bilirubin or hematin is impaired.