The sex of a person with malformation of the external genitals and malformation or absence of the internal organs of generation may be difficult to determine. In recent years it has not been unusual to perform a biopsy on the gonads (at times necessitating laparotomy) to arrive at a valid conclusion. In two instances of malformation we have definitely established the sex of the patients by demonstrating the presence of the female sex hormone in the circulating blood by injecting lipoid extract of the blood into castrated mice and thereby producing a positive (estrual) vaginal spread.1 The theoretical considerations involved are that a person possessing functioning ovaries, irrespective of whether the tubular tract (vagina, uterus, tubes) and external genitals are malformed or aplastic, should have a demonstrable quantity of female sex hormone in the circulating blood some time between the fifth and fifteenth days after ovulation has taken place.