Normal tissue reactivity on frozen sections was examined with 4 monoclonal antibodies (MAbs) that were reported previously to be negative or weakly reactive with normal tissues, and strongly reactive with some types of carcinoma. All 4 antibodies reacted strongly with certain normal epithelial cells. The antibodies tested include 2 antibodies to ovarian cancer, MOv18 and MOv19, one antibody to breast cancer, H23, and one antibody reactive with a range of carcinomas, B72.3. MOv18, MOv19 and H23 reacted with many normal glandular and ductal epithelial cells, while B72.3 reacted most strongly with secretions of the small intestine epithelium and the suprabasal squamous epithelial cells of the esophagus. Since the tissue distribution of MOv19 was very similar to that of another antibody described previously, MW207, these two antibodies were compared by competitive binding inhibition, and found to recognize the same epitope. Our data emphasize the importance of repeated, independent tests of antibody specificity.