The synthesis of α‐fetoprotein and other fetal plasma proteins was studied by perfusion of isolated human fetal livers with an artificial medium containing leucine‐1‐14C. The rates of release by fetal liver of α‐fetoprotein and albumin were analyzed by measuring the changes in their concentrations in the perfusion medium after the first hour of perfusion. The release of α‐fetoprotein from the liver of a 14‐week fetus was 19 μg/min. At 17 weeks of gestation, the rate was 26 μg/min and, at 20 weeks, 21 μg/min. The corresponding values for albumin were 50 μg/min, 92 μ/min, and 196 μ/min. Within the same period of gestation, the plasma concentration of α‐fetoprotein decreased and that of albumin increased with advancing fetal age. The results suggest a more rapid turnover of α‐fetoprotein than albumin in vivo. The incorporation of leucine‐1‐14Cinto serum proteins was determined by autoradiography after electrophoresis on cellulose acetate, immunoelectrophoresis, or immunodiffusion. Radioactivity was found in α‐fetoprotein, albumin, α1‐antitrypsin, Gc‐protein and α‐lipoprotein — indicating hepatic synthesis of these proteins. No radioactivity was found in IgG present in the perfusion medium, and no haptoglobin was found in the medium.