Among the most recent methods for investigation of proteins in biological fluids SDS Polyacrylamide-gel-electrophoresis and Isoelectrofocusing (IEF) have recently been introduced into laboratory practice. The present investigation has been performed on 20 samples of amniotic fluid obtained during normal pregnancies in the first and in the third trimester. The obtained results suggest that IEF analysis seems to have a selective advantage in allowing the separation of bands which can not be easily recognized with SDS electrophoresis. These bands detected by IEF and present in amniotic fluid during late pregnancy seem to be related to some low 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 A.F. obtained during pregnancies complicated by infections.