Fragments of human foetal organs and blood at 5-11 weeks of postfertilization development were cultured in radioactive protein precursors. The secreted products were characterized by immunoprecipitation, and by measuring the mobility of the immunoprecipitates on polyacrylamide gels. It was found that secondary human yolk sacks secreted apolipoproteins A1 and B. The work of previous authors on the synthesis of other serum products by this organ and by the foetal liver and by the foetal intestines was confirmed. Within the yolk sack, the endoderm, the blood cells, and the outside epithelium reacted with antibodies against apolipoprotein A1 and transferrin. By metabolic labelling of umbilical cord blood, it was found that blood did not secrete apolipoproteins A1 and B. Blood cells could therefore not be a source of these secreted products.