Among the most recent methods for investigation of proteins in biological fluids SDS polyacrilamide-gel-electrophoresis (SDS-PAGE) and isoelectrofocusing (IEF) have recently been introduced into laboratory practice. The present investigation was performed on 14 samples of amniotic fluid obtained during the third trimester in pregnancies complicated by Rh-sensitization and polyhydramnios in which protein concentration was higher than in normal pregnancies. The obtained results suggest that IEF analysis seems to have a selective advantage in allowing the separation of bands which cannot easily be recognized with SDS electrophoresis. These bands detected by IEF and present in amniotic fluid during late pregnancy seem to be related to some molecular weight lipoprotein fractions and we suggest that they might be used as a possible marker for monitoring fetal lung maturation. In conclusion we think that it would be of great interest to evaluate the usefulness of IEF analysis in examining amniotic fluid obtained during pregnancies complicated by polyhydramnios and Rh-sensitization.