Comparative tests carried out in human groups under laboratory conditions failed to show any significant difference in the immunizing efficiency of high (450th) and low (212th) culture passage material of yellow fever virus 17D, when comparable dosages of virus were used. With dosages of virus approaching 100 m.l.d. for mice, some individuals fail to produce demonstrable antibodies. Other persons, however, show definite immunity after as little as 10 m.l.d. for mice of either high or low subculture virus.