Monoclonal antibodies (Mab) were prepared against nonspecific cross-reacting antigen (NCA) and were selected on the basis of their absence of reactivity with carcinoembryonic antigen (CEA). Four Mab were found which allowed the characterization on CEA of three epitopes, defined A, B, and C. These epitopes were all located on the peptidic moiety of this highly glycosylated antigen and were present on NCA molecules of heterogeneous m.w. (greater than 100,000, 80,000, and 48,000 m.w., the latter being the most abundant). The amount of NCA was estimated in 251 human sera both by a conventional RIA, using a rabbit antiserum, and by EIA, using different Mab: Mab 4, 18, and 33, which reacted, respectively, with epitopes A, B, and C. Each assay gave a different value of the absolute concentration of NCA in the serum. On the whole, Mab 4 gave lower values, whereas Mab 18 and 33 gave higher values as compared to RIA. Furthermore, whereas all of the human sera contained NCA which was measurable by RIA, 67 sera typed negative in EIA when using Mab 4 or 18. Eight additional sera were negative in more than one EIA. Negativity when using Mab 33 was observed in only one serum, which was also negative with Mab 4 and 18. Twenty-five of 30 sera which were negative with Mab 4 came from cancer patients, and 32 of 37 sera negative with Mab 18 came from normal subjects and noncancer patients, giving a statistically highly significant difference between the two groups of sera (p less than 0.001). Analysis of tissue perchloric extracts and NCA samples purified from these extracts gave similar results. Three extracts (one from lung, two from cancer tissue) and the corresponding NCA samples were negative with Mab 18. The discrepancies observed in these assays are best explained by assuming the existence of antigenic variants of NCA which have not been described previously. These variants appear to exist in various proportions in the different sera. The variants may represent antigenically complete and incomplete molecules. Alternatively, most of the NCA molecules may be incomplete, lacking one or another of the several NCA-specific epitopes. Sequential immunoprecipitation experiments were in favor of the second hypothesis, showing that most of the NCA molecules were incomplete, lacking either epitope A or B.

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