Liver sinusoids contain marginated mononuclear cells which are highly cytotoxic against several tumor cell lines. These cells may play an important role in the elimination of tumor and viral antigens of enteric origin. The question arises whether they are bloodborne, temporarily resident, replacable or permanently resident cells. In order to obtain the answer, a cytogenetic analysis of sex chromosomes in the mononuclear cells from a liver sinusoidal washout of male Lewis rats who had received liver transplants from syngeneic female donors was performed 4 weeks after grafting. About 9% of the liver sinusoidal mononuclear cells obtained from both control and transplanted livers showed mitotic activity, whereas no dividing cells could be found in peripheral blood. In all experiments karyotypic studies of these dividing liver sinusoidal mononuclears from female livers transplanted to syngeneic males revealed in all experiments the male (XY) karyotype. This finding indicates that the cells under study were of blood origin and are not a constitutive cellular component of the liver. They are probably recruited by the liver and divide locally at a signal of a so far unknown origin.