Alkaline phosphatase of the placental type in serum has been suggested as a "marker" for malignancy and pregnancy. We describe a highly sensitive enzyme-linked immunosorbent assay (ELISA) for the measurement of this enzyme in serum and ascitic fluid. The assay will detect as little as 0.4 microgram/L, significantly less than with a radioimmunoassay performed with the same reagents. It is highly specific; it does not measure even above-normal concentrations of the intestinal and liver isoenzymes of alkaline phosphatase. The assay is technically simple and allows the processing of many samples in less than 10 h. We measured this isoenzyme in serum of an adult control population. The upper limit of normality is 1.85 microgram/L. Interference by rheumatoid factor was eliminated. Concentrations of the analyte were increased in all pregnancy sera tested. Concentration and activity as measured by two different catalytic assays correlated well. Samples from cancer patients also showed a good correlation, with some exceptions. Possible reasons for these exceptions are discussed. The high sensitivity, specificity, and simplicity of this assay should make it a useful adjunct in monitoring cancer and pregnancy.