Plasma/serum proteins of fetal blood samples (N = 88) obtained under ultrasound guidance between the 18th and the 39th week of pregnancy, of blood samples collected from premature infants (N = 19), newborns at term (N = 20) and children of less than 5 years of age (N = 55) were analysed by high-resolution two-dimensional polyacrylamide gel electrophoresis. By comparison with adult 'reference' protein maps, tens of different proteins (and some of their genetic variants) were identified on the electrophoretograms. After the 18th week of gestation, albumin, transferrin, Factor B, glu- and lys-plasminogen, antithrombin III, Gc-globulin, alpha 1-antitrypsin, alpha 2-HS-glycoprotein, several apolipoproteins (apo A-I, A-II, A-IV, C-II, C-III, D, E, J), retinol-binding protein, transthyretin and alpha-fetoprotein could be observed. During intrauterine life, the size of the spots corresponding to alpha-fetoprotein progressively decreased, whereas the protein pattern globally showed an increase in the number and in the size of the spots. These modifications were particularly apparent in the regions of the electrophoretograms restricted to the heavy and light chains of IgG and to alpha 1-antichymotrypsin. In addition, we observed an unidentified fetal polypeptide characterized by an apparent molecular weight (M(r)) of 46 kDa (P46) and a pI of 5.0. P46 was present in all fetuses and all infants of less than 2 years of age.(ABSTRACT TRUNCATED AT 250 WORDS)