In 1930 the determination of serum alkaline phosphatase in patients with bone or liver disease ushered in the era of clinical enzymology. The association of elevated (bone) alkaline phosphatase in serum of patients with osteogenic sarcoma was the first evidence that tumor cells themselves produced the enzyme. It became clear, however, in the 1960s that the serum alkaline phosphatase was not a single enzyme but consisted of a family of isozymes originating from liver, bone, intestine, and placenta. This was a consequence of the introduction of a combination of electrophoretic separations, heat inactivation, and organ-specific amino acid inhibitors. This combination of measurements made possible the demonstration of a serum alkaline phosphatase of lung cancer origin, as confirmed by the histochemical visualization in lung cancer of the Regan Isozyme (placental alkaline phosphatase-PLAP). At present, the measurement of PLAP has its greatest utility as a tumor marker in seminoma and ovarian cancer. A PLAP-like isozyme in normal testis and ovary is expressed in these and other neoplasias and appears to be related to rare alleles of placental alkaline phosphatase. Current studies have utilized a panel of monoclonal antibodies to detect useful epitopes that suggest that PLAP and PLAP-like isozymes are coded by different genes. The PLAP gene has now been cloned and sequenced by Millan and others. This fundamental new information is providing a base line that will make it possible to explain the overlapping specificities of intestinal and placental isozymes, the degree of uniqueness of the PLAP-like isozyme, the precise mechanism of uncompetitive inhibition by L-phenylalanine and the evolutionary history of the alkaline phosphatases.(ABSTRACT TRUNCATED AT 250 WORDS)