Plasma incompatibility was investigated as a possible factor in transfusion reactions in dogs. Urticaria, failure to retain within the circulation the equivalent of transfused plasma volume and protein, and increased gastric acid secretion almost invariably followed transfusion of plasma from another dog but not transfusion of the dog's own plasma. The cutaneous reactions were abolished and the retention of plasma was improved by administration of an antihistamine. Erythroeyte isoagglutinins did not appear to be involved in these reactions. 79