Drugs are a rare cause of immune hemolytic anemia, but an investigation for a drug antibody may be warranted if a patient has definitive evidence of immune hemolysis, other more common causes of hemolysis have been excluded, and there is a good temporal relationship between the administration of a drug and the hemolytic event. Drug antibodies are either drug-dependent (require drug to be in the test system) or drug-independent (reactive without drug present in the test). Drug-dependent antibodies are investigated by testing drug-treated red blood cells (RBCs) or by testing RBCs in the presence of a solution of drug. Drug-independent antibodies are serologically indistinct from idiopathic warm autoantibodies and cannot be defined or excluded by serologic testing. Nonimmunologic protein adsorption, caused by some drugs, is independent of antibody production but may also cause immune hemolytic anemia. Serologic methods for testing for drug antibodies are presented, and observations from more than 30 years of this laboratory's experience are discussed.