Mosaicism of ornithine transcarbamylase expression was immunohistochemically examined in hepatocytes of spfash heterozygous female mouse livers. An immunohistochemical method using polyclonal antibodies against ornithine transcarbamylase visualized the mosaicism in serial paraffin sections. Very complicated mosaic patterns consisting of positive and negative hepatocytes were obtained in a section of adult liver, but a computer-aided three-dimensional analysis of serial sections demonstrated that patches with complicated shapes and various sizes, which are contiguous groups of positive or negative hepatocytes and are isolated in sections, connected very well with one another. No definite orientation such as portal-central was observed in the three-dimensional images of each patch. In balanced regions of the mosaicism, most individual plates along their straight portal-central lengths did not appear to be composed of only marker-positive or marker-negative hepatocytes. By contrast, in unbalanced regions of the mosaicism, individual plates along their total portal-central lengths often consisted entirely of a major type of hepatocytes. A patch appeared to be present in more than two lobules, but each patch did not constitute a complete lobule. Complicated mosaic patterns of patches were also seen in neonatal and postnatal livers. These results suggest that, although hepatocytes proliferate and migrate extensively during development, they might allocate their daughter cells contiguously and the orientation of their allocation might be random, leading to the formation of three-dimensionally large contiguous quasiclones of hepatocytes, the shapes of which are very complicated.