The X-linked testicular feminization mutation (Tfm) in the mouse is characterized by an androgen receptor defect. Due to random X-chromosome inactivation, XTfm/X+ heterozygotes are mosaics with respect to Tfm. They are composed of androgen receptor deficient XTfm cells and normal X+ wild-type cells. If Tfm heterozygotes are converted to XX males by the sex reversal factor (Sxr) the mosaicism is expressed. Therefore in sex reversed Tfm heterozygotes (XTfm/X+-Sxr) intersexual sex organs develop. In five intersexes with small male accessory glands and hypospadia and one heavily feminized intersex with vagina and caudally dislocated deferent ducts the mosaic is visualized by 3H-DHT-autoradiography. In the epididymis differentiated wild-type cells show nuclear labeling, whereas undifferentiated Tfm cells are unlabeled. Unlabeled Tfm cells are also encountered in the vesicular glands of the heavily feminized animal, demonstrating that Tfm cells can participate in the formation of male sex glands. The urethral glands of the mosaic animals are composed of unlabeled Tfm lobules exhibiting the female phenotype of the glands, and of labeled wild-type lobules exhibiting the male phenotype. Formation of a vagina and deviation of the deferent ducts is correlated with lack of androgen binding sites in the connective tissue.