The identification of AFP in the serum after birth is always due to a hepatoma or, more rarely, a teratocarcinoma. The increasing sensitivity of methods of immuno-chemical investigation and radioimmunoassay, permit today its identification in cases other than hepatoma. In the foetus, the hepatocyte or its precursor is the site of main synthesis but is not exclusive. The synthesis of AFP has been demonstrated both in the cells of endodermic origin, yolk sack, anterior part of the intestine, stomach and colon. In hepatomas, only a small proportion of neo-hepatocytes is AFP positive on immunofluorescence or on electron microscopy. A quantitative study of AFP is useful above all: a) During pregnancy: detection of foetal distress or malformation when AFP is present in the amniotic fluid. b) For the supervision of patients after surgical treatment. The interest of AFP detection and estimation using high sensitivity methods is emphasized.