Alpha-fetoprotein (AFP) is a specific glycoprotein which is synthesised in the fetal liver and released into the blood stream together with the closely related protein, albumin. It has been proposed that AFP functions as a carrier of essential fatty acids to certain developing cells and as a possible immunosuppressor. In man its synthesis is under the strict and complicated control of transcription of a single gene on chromosome 4. The concentration of AFP in fetal serum is greatest at about 13 weeks gestation and then decreases up to birth. During pregnancy AFP passes into the amniotic fluid and also across the placenta, so that the concentration of AFP in maternal serum increases during pregnancy in a characteristic way. Greater than normal increases may indicate certain pathological states in the fetus. Serum concentrations of AFP in the newborn infant decrease rapidly to reach levels typical for adults (< 10 micrograms/L) usually by the end of the first year. Raised concentrations of serum AFP appear in a large proportion of patients with primary hepatoma and in a smaller percentage of patients with other malignant diseases (tumours of the testis, ovary, bronchi, gastrointestinal tract). In addition, increases in serum AFP are found in other illnesses accompanied by damage to hepatocytes in the liver (hepatitis, cirrhosis etc.). Certain differences in the structure of the oligosaccharide portion of the molecule have been shown between AFP synthesized by benign or by malignant cells and between AFP synthesised by hepatocytes or by cells of endodermal origin. These differences have been used as an aid in the diagnosis of liver diseases where serum AFP is elevated. Since AFP is not strictly specific for a certain type of carcinoma, its determination is primarily used in medicine for monitoring the effects of therapy and surgery on the course of malignant conditions which initially showed increased levels of serum AFP.