Sex-chromatin of amnion cells was determined in the placentas of 3029 newborns. Results were compared with phenotypic characteristics. Among 1563 newborn males only one sex chromatin positive case was found. This newborn was an anencephalic. No sex-chromatin negative females were found among 1466 newborns tested. The frequency with which a discrepancy occurs between the morphological sex of the cell nucleus and phenotypic sex characteristics is considerably less according to these findings than the results in the literature.