PIP: Synthesis of serum alpha fetoprotein (AFP) was studied in 16 human embryos and fetuses from 4.2-18 weeks of gestation by incubation of selected tissues in radiolabeled amino acids followed by immunoelectrophoresis of the culture fluids and radioautography. Relatively large amounts of radioactive AFP, judged by relative intensity of AFP precipitation line on radioautography, were found in each of the liver cultures of the developing yolk sac. AFP was observed in smaller amounts in almost all gastrointestinal tract cultures studied. Labeled AFP formed in kidney cultures from 1 of 9 conceptuses and in only 1 of 14 placentas cultured. None of the cultures containing lung, thymus, pancreas, skeletal muscle, amnion, chorion, or blood produced detectable amounts of AFP.