The presence is recorded of an abnormal hemolysin in the serum of a patient with severe acquired hemolytic anemia. Its activity in vitro was determined by the pH of the corpuscle-serum suspension; the optimum pH was about 6.8 to 7.0 and there was inhibition above pH 8 and below pH 6. This pH range is contrasted with that of other human serum hemolytic systems; it is similar to that found in nocturnal hemoglobinuria.

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