It is widely accepted that the syndrome of pseudomembranous enterocolitis is due to a Staphylococcus enteritis secondary to administration of wide-spectrum antibiotics. However, in a previous paper 1 we showed that it is possible to reproduce consistently the pathology and many of the clinical manifestations of this syndrome in dogs (Figs. 1-8) in the absence of antibiotics or bacterial infection. This was done by means of an intra-aortic injection (through polyethylene catheters inserted in the femoral artery) of incompatible blood. There resulted an episode of intravascular clotting with the production of fibrin thrombi in the capillaries of the mucosa and submu cosa of the bowel. This caused necrosis (due to ischemia) of the superficial mucosa and pseudomembrane formation. It has since been possible consistently to prevent the syndrome by preheparinizing the dogs. It was found that the lesions in the jejunum, ileum, and colon could