The unusual presentation of a factory worker with severe hemolytic anemia which remitted following splenectomy prompted a search for an environmental cause for red cell injury. The investigation showed the presence of an immunoglobulin in the patient's serum and on the red cells and small amounts of complement on red cells. The patient's serum caused agglutination of a normal person's red cells only when dieldrin-coated, a reaction blocked by first reacting the serum with dieldrin. The spleen of the patient had a greater than normal concentration of dieldrin, the source of dieldrin being dietary. It is concluded that dieldrin became immunogenic and provoked a chemical immunohemolytic anemia. The spleen played a major role in destruction of red cells injured by the immunopathic process and in accumulation of the antigenic substance dieldrin.