Summary.-Double-antibody radioimmunoassay using a mouse monoclonal anti-CEA (MA/1) has been used to measure CEA in human serum. Low levels of MA/1 -binding CEA have been found in serum from normal individuals and moderately raised levels are sometimes associated with certain non-malignant diseases. As with conventional anti-CEA, the MA/i antibodies can recognize significant amounts of CEA in serum from patients with a variety of solid tumours. However they appear to recognize a different immunodeterminant and possibly a different population of CEA molecules to, or a subset of, those measured by two routine assays. Studies in which the MA/i assay was directly compared with the results of the Charing Cross routine and Abbott EIA assays have indicated that different immunological forms of CEA may be expressed in the course of tumour progression but no prognostic value was evident in this study. Our results stress the need to resolve immunological specificities expressed by CEA-like molecules and evaluate their clinical importance.

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