The ontogenesis of alpha-fetoprotein (AFP) was studied in the C57BL, CC57BR, C3H/He, A/He and DD mice during the first 4 weeks of life. The AFP hepatocytes were determined by indirect immunofluorescent staining of liver cells isolated with sodium tetraphenyl borate; the content of AFP in blood was determined by the method of rado immunodiffusion. The methods utilized allowed to obtain the quantitative characteristics of the dynamics of AFP-positive cells in the liver and the content of AFP in the blood. In the newborn mice over 90% hepatocytes contain AFP, the intensity of luminescence being heterogenous. The number of bright-liminescent cells equals to 50% during the first day of life rapidly decreases and none of them are found by 9--11 days. The number of average- and weak-luminescent hepatocytes does not decrease during the first 10 days, but then gradually decreases and none of them are found by 23 days in the CC57BR and by 27 days in the DD and A/He mice. A comparison of the dynamics of AFP-positive cells in the liver and the content of AFP in the blood has shown that the bright-luminescent hepatocytes are the main producent of this protein in the early postnatal ontogenesis.