From the University Departments ofImmunology and *Medicine, Royal Liverpool Hospital, PO Box 147,

SUMMARY By means of enzyme immunoassays based on two monoclonal antibodies with specificities for distinct forms of placental type alkaline phosphatase (P1-ALP), together with L-leucine inhibition studies, it has been possible to distinguish the Nagao type carcinoplacental enzyme from other placental type alkaline phosphatases. This approach has shown that it is the Nagao type (placental like) enzyme which is detectable in small amounts in the plasma of healthy individuals, particularly cigarette smokers. Human placental type alkaline phosphatase (Pl-ALP) is a thermostable genetically polymorphic isoenzyme of placental trophoblast membranes. ' 2 It was designated a carcinoplacental protein when identified also in the circulation of certain patients with cancer.34 P1-ALPs derived from tumour cells form at least two distinct groups: the Regan type enzyme is closely similar or identical to placenta derived P1-ALP, ' I and the Nagao type enzyme is