The authors investigated the effects of DPH to the RBC osmotic fragility of normal person in vitro and obtained the result as follows : The usual doses, 20 ug of DPH, were added to four persons’ RBC suspension in various concentration of buffered NaCl in orders and divided evenly in half respectively, the one for incubation at 37℃ and the other for control omitting the incubation. There was noted a remarkable decrease in the RBC osmotic fragility with incubation comparing to the decrease in the RBC osmotic fragility without incubation.