FOR a number of years it has been known in this laboratory that haemolysis of moderate degree may occur when red cells are being treated with the viruses of mumps and Newcastle disease. This is not characteristic of any of the influenza viruses. The haemolysis was of slight degree and apparently irregular in its occurrence. Following the appearance of the paper by Morgan, Enders and Wagley1 on mumps virus haemolysis, it was noted that when dilutions of Newcastle disease virus were made in phosphate solution instead of physiological saline, both fowl and human red cells were more extensively lysed. A comprehensive study of the phenomenon was afterwards begun.