A patient with renal disease developed Coombs-positive hemolytic anemia while receiving cephalothin therapy. An anti-cephalothin IgG antibody was detected in the patient’s serum and in the eluates from her erythrocytes. In addition, nonimmunologic binding of normal and patient’s serum proteins to her own and cephalothin-coated normal red cells was demonstrated. Skin tests and in vitro lymphocyte stimulation revealed that the patient was sensitized to cephalothin and also to ampicillin. Careful investigation of drug-induced hemolytic anemias reveals the complexity of the immune mechanisms involved.