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Alpha1-fetoprotein (AFP) is an alpha1-glycoprotein which can be found in high concentration during fetal development in many mammals, birds, sharks and, also, man. The alpha-fetoproteins of various species have similar physico-chemical properties and often common antigenic determinants. Differences of microheterogeneity depend on a different content of sialin-acid. During human fetal development the serum AFP concentration falls with increasing gestational age. 4-5 weeks after birth AFP can be detected usually in low serum concentrations. Using more sensitive immunulogic techniques e.g. radioimmunoassay there was shown that AFP is present in sera of normal adults in concentrations of 10-20 ng/ml. AFP serum concentrations rise physiologically during pregnancy up to 500-550 ng/ml. During fetal development liver, yolk sac and gastrointestinal tract are the major sites of synthesis. In primary liver cell carcinoma, hepatoblastoma and in teratoblastoma containing yolk sac tissue AFP synthesis rises in tumor cells; the AFP serum concentration increases above 2 microgram/ml. In patients with benign liver diseases e.g. virus hepatitis, a transient rise of AFP serum concentrations was seen. Moreover, increased levels of AFP were found in hereditary diseases e.g. congenital tyrosinemia, ataxia-telangiectasia and in the amniotic fluid in congenital nephrosis of Finnish type. AFP assay in serum is clinically important for the control of course and treatment of primary liver cell carcinoma and teratoblastoma. AFP assay in amniotic fluid is a method for the prenatal detection of neural tube defects and the fetal distress syndrome, especially.