X-chromosome inactivation patterns were investigated in livers of nine spfash female heterozygous ornithine transcarbamylase (OTC)-deficient mice. Quantitative morphometric analysis of cellular mosaicism was performed on sections of frozen liver reacted with purified anti-OTC antibody and prepared for immunofluorescent microscopy. Analysis of enzymatic OTC activity was performed on sections of these livers using a radiochromatographic technique. Several areas of cellular mosaicism were seen in each of the histological sections that were studied. The distribution of the volume fraction of the liver tissue cells having cells with normal OTC content among the nine mice ranged from 20 to 70% and it correlated (r = 0.8, P = 0.005) with the enzymatic activities of the respective livers. The extreme variegation of mosaic patches in the liver suggests the high probability that a single needle biopsy will be diagnostic in females heterozygous for an OTC mutation. This study also suggests that at the time of X inactivation, the number of primordial liver embryonic cells is small and the observed variegation of liver mosaicism probably results from complex migration patterns of liver cells during fetal development. This study shows that the spfash mouse is a suitable animal model for quantitative studies of X-chromosome inactivation in liver using immunohistochemical staining of OTC protein.