Using indirect immunofluorescence, radial immunodiffusion and radioimmunoassay methods, changes in the number of AFP-synthesizing cells in the liver and in the serum AFP concentration during postnatal development were studied in C3H/HeSto and BALB/cLacSto mice (normal and nu/nu (nude) mutants). From the age of 2 weeks, serum AFP levels were significantly higher in nude mice as compared to normal mice. AFP-positive cells also disappeared later from the liver of nude mice compared to normal. Morphological analysis and liver smear cell counts demonstrated that hemopoietic foci persist in the liver of nude mice during their entire life. The relationship between AFP synthesis and hepatic hemopoiesis is discussed.