Transfusion reactions can have immunologic or nonimmunologic causes and may be immediate or delayed. Immune-mediated hemolytic reactions result from antibodies to surface antigens of transfused erythrocytes. Canine blood is grouped according to these surface antigens. Typing is currently available for six of these antigens. A canine universal donor is negative for all of these dog erythrocyte antigens (DEAs) except DEA 4. Universal donors should be used when possible; other donors can be used if they are known to be compatible with the recipient. Other possible immunologic reactions include acute hypersensitivities mediated by IgE antibodies and leukocyte or platelet sensitivity caused by recipient antibodies to the donor's white cells or platelets. Nonimmunologic reactions have a wide variety of mechanisms. The type and severity of clinical signs vary according to the specific reaction. Part II of this two-part presentation discusses the prevention and treatment of transfusion reactions.