Apolipoproteins were determined in 50 human amniotic fluids obtained by amniocentesis during weeks 16 and 22 (n = 26) or 33 and 41 (n = 24) of gestation. Whereas apo A-I, A-II, A-IV, and E were identified at levels of 1, 0.7, 0.8 and 1% of normal human adult plasma, respectively, apo B levels were only 0.04% of plasma concentration and apo C-III levels were below the detection limits of the assay. Amniotic fluid levels of apo A-II, A-IV and E did not differ between early and late stages of pregnancy, but levels of apo B decreased and apo A-I increased significantly in late pregnancy. Isofocusing showed apo A-I, A-II and A-IV in identical positions as compared to human adult plasma. Furthermore the known genetic polymorphism of apo A-IV was detectable. Individuals heterozygous, for the variant form apo A-IV-2, showing the phenotype apo A-IV (2-1), had significantly higher levels of apo A-I and A-II as compared to the common phenotype apo A-IV (1-1). We conclude that human amniotic fluid contains the major plasma apolipoproteins at about 1% of plasma levels with the exception of apo B which shows a level at an order of magnitude less than high-density lipoprotein apoproteins in comparison to their plasma counterparts.