Fatal hemolytic anemia developed in a 52-year-old woman who was treated with a cephalosporin, ceftriaxone. The patient's red cells (RBCs) were coated with C3, but no RBC-bound IgG, IgA, or IgM was detected. Her serum contained an antibody that did not react with cephalosporin- coated RBCs but reacted strongly with RBCs in vitro when her serum was added to drug and RBCs. This is the first case of immune hemolytic anemia associated with ceftriaxone, the first case of fatal cephalosporin-induced hemolytic anemia, and the second case in which a cephalosporin antibody showed in vitro and in vivo characteristics usually thought to be associated with the so-called immune complex mechanism.