Summary1. Amniotic fluid surrounds the human fetus and is separated from the uterus by the amnion, chorion and placenta. The ability to obtain samples of amniotic fluid from women by a simple procedure has encouraged studies on the nature and origin of the fluid, and on its use for the diagnosis of a variety of clinical conditions. The fluid contains cells, which are of fetal origin, and can be grown in a tissue culture. Cyto‐genetic and biochemical analyses can therefore be used to detect chromosomal aberrations and inborn errors of metabolism in the fetus.2. The supernatant of amniotic fluid contains many of the solutes typical of extracellular fluid. In particular, it contains a wide range of proteins and those which are of fetal origin are likely to be of use in the prenatal diagnosis of fetal disease. This review examines the nature and origin of the soluble protein in amniotic fluid, and discusses the diagnostic uses of the proteins which are of fetal origin.3. In other mammals, the arrangement of the fetal membranes is different from that in man, and these differences are reflected by changes in the nature of the amniotic fluid. Thus data from other animals have little applicability to man.4. Electrophoresis and immunoelectrophoresis have established that the major proteins in amniotic fluid are also present in maternal and fetal sera. Their concentrations in the fluid are influenced by their molecular weight and proteins larger than about 2.5 times 106may be excluded. Towards term, phenotyping studies show that a number of serum proteins in amniotic fluid are of maternal origin. In the case of group‐specific component (Gc) this has been shown to be so throughout pregnancy. Such proteins must enter the fluid by diffusing across either the chorion or the chorionic plate and then the amnion.5. It has been previously claimed that various serum proteins in amniotic fluid are of fetal origin. For albumin and IgG there are data that strongly support a maternal origin. The evidence on the origin of insulin is inconclusive. The concentration of β2‐microglobulin in amniotic fluid exceeds that in maternal serum and is probably too high also for fetal serum to be its major source. It has a wide tissue distribution and probably enters the fluid from surrounding structures.6. Alpha‐fetoprotein in amniotic fluid is of fetal origin as it is present in maternal serum at far lower concentrations. It is found in fetal serum, urine and yolk sac, but it is not clear how it enters the amniotic fluid of normal fetuses. The concentrations of Gc and alpha‐fetoprotein have been measured in amniotic fluid and in their sera of origin. The relative concentration of Gc in amniotic fluid was found to be much greater than that of alpha‐fetoprotein and the concentration gradients of these marker proteins can be compared with data for other proteins. In this way further evidence has been obtained that the albumin, α1,‐antitrypsin and transferrin in amniotic fluid are mainly of maternal origin throughout pregnancy.7. Immunological studies have shown that at least three proteins of non‐serum origin are present in amniotic fluid and they have also been located in the amnion and uterine decidua.8. The enzymes present in amniotic fluid are summarized. Many lysosomal enzymes are clearly of fetal origin since they show altered specific activities in the appropriate cases where the fetus is affected with an inborn error of metabolism. For other enzymes, analysis of specific activity gradients can help to decide the extent to which an enzyme is of serum origin, although this will not exclude the possibility of a maternal (uterine) contribution. The results of such analyses suggest that, relative to the serum protein in amniotic fluid, the greatest concentrations of the minor non‐serum proteins in the fluid occurs between thirteen and eighteen weeks of pregnancy and also towards term.9. Some inborn errors of metabolism may be diagnosed prenatally by measuring the specific activity of the respective enzyme in amniotic fluid. However, the presence of different enzymes with similar substrate specificities has prevented this in Pompe's disease.10. In cases where the fetus is affected with anencephaly or spina bifida there is an increase in the concentration of alpha‐fetoprotein in the amniotic fluid. This has provided a way of detecting these diseases early enough to allow termination of pregnancy.11. The discovery of new proteins in fetal serum and in the tissues surrounding the amniotic cavity would seem to provide the best chance of extending the uses of amniotic fluid into the other areas