Human amniotic fluid contains a complex mixture of proteins, of which only the minority are of fetal origin. We have identified the fetal polypeptides of second trimester amniotic fluid samples by two different methods. 2. The first method was the side by side comparison of silver-stained two-dimensional polyacrylamide gels of amniotic fluid polypeptides with pregnant female plasma polypeptides, after passage of both through a Blue Sepharose affinity column to remove albumin. The second method was the identification of the fetal polypeptides in two-dimensional Western blots with an antiserum made specific for fetal proteins. 3. Using these techniques we have identified 13 major fetal polypeptide fractions with apparent molecular weights of 220, 200, 82, 70, 59, 52, 50, 36, 30, 25, 20, 18 and 11 kDa. Five of these polypeptides, with molecular weights of 82, 59, 50, 20 and 18 kDa, have not previously identified. The identification of these fetal components provides a reference base for molecular studies of normal and pathological fetal development.
