In this paper the relation between Arakawa's reaction and the chlorine content of the urine of breast-fed infants is investigated. The chlorine content in the urine of breast-fed infants is 38.0 mgms. % in Arakawa-positive cases and 41.5 mgrms. % or 37.3 mgrms. % in inter-mediate or completely or almost completely negative cases. And viewed from the chlorine figures of milk of mothers with different Ara-kawa's reaction and those in the urine of breast-fed infants nursed with milks of different Arakawa's reaction, it is probable that an ab-normal chlorine retention occurs in the body of babies fed with Ara-kawa-negative milk; more so, if Umemura's opinion is taken into con-sideration.