Glutathione S-transferase B1 (GST B1) concentration in blood and amniotic fluid from fetuses investigated for a variety of conditions including rhesus (Rh) allo-immunisation was assessed for its usefulness as a measure of liver damage caused by hypoxia in utero. The concentration in blood from the intrahepatic vein (IHV) was 10-fold higher than that from the placental cord insertion suggesting that parenchymal liver cells are damaged during blood sampling from the IHV. As a measure of hepatocellular impairment caused by intra-uterine hypoxia, levels were higher in frankly acidotic fetuses than in normally managed Rh fetuses. The degree of hypoxia required to trigger the release of GST B1 into the plasma remains unclear.