A comparative study was performed of the polyacrylamide gel electrophoretic patterns of proteins in 100 sera from the umbilical cord and maternal sera after normal pregnancies and deliveries. To simplify this analysis, the electrophoretic pattern was arbitrarily divided into nine regions that were studied according to the number of band frequency and colour intensity. Protein nomination was made with literature data. Total protein concentrations in both sera were determined by colorimetric techniques. The most important data obtained from umbilical cord serum pattern were the following: a greater number of bands in the post albumin, post beta and gamma 1 regions; absence of "hemopexin" band or faintly stained if present; abundance of 7s-globulins with lack or scarcity of "haptoglobin" bands in gamma 2 region in 100 per cent of gels; and finally, the band closer to the origin (B-lipoprotein) slightly stained. Minimum number of bands was 11, maximum 24. Features of maternal serum were: highly stained "hemopexin" in 100 per cent of gels; frequent band absence in gamma 1 region; in gamma 2 region, there are scarce 7s globulins and "haptoglobins" are numerous and highly stained; in the middle part of this same region, frequent presence of a band that is more coloured than "haptoglobins"; the three last bands close to the origin are always observed showing high staining. Minimum number of bands was 13 and maximum 30. Total protein average concentration was 6.28/g/100 ml in umbilical cord serum and 7.14/g/100 ml in mother serum. Comparison of electrophoretic patterns in 100 paired sera showed that only three were identical. In this paper we discuss some of the possible causes of differences observed between both sera.