Summary. Because of the high mortality of human XO embryos, it was questioned whether the single X in a proportion of cells was inactivated. If genetic inactivation of the single X in XO conceptuses was of significant importance for reduced viability and subsequent abortion, a proportion of cells grown from XO fetuses should carry an inactivated late-labelling X chromosome. This hypothesis was tested by autoradiography and analysis of the late DNA-labelling pattern of XO cells derived from a 15-week-old spontaneous human abortus. None of the analysed cells had a late-labelling X chromosome. Consequently, the XO cells from this aborted fetus were not different from XO cells from liveborn individuals.