The polymorphic epithelial mucin (PEM) appears to be the target molecule for many monoclonal antibodies (MAbs) which react with tumour-associated and epithelium-specific antigens. PEM contains a large domain made up of 20 amino-acid tandem repeats which are highly immunodominant as many of the antibodies reactive with this molecule recognize epitopes within this area. Using overlapping peptide octamers, we have precisely mapped the epitopes of 4 MAbs reactive with the tandem repeats including one, SM-3, which shows enhanced tumour specificity. We report that the core of the SM-3 epitope corresponds to the continuous amino acid sequence Pro-Asp-Thr-Arg-Pro. We also show that the epitopes recognized by 3 other antibodies, which show reactivity with normal and malignant tissues, map to within this area of the tandem repeat. However, none of these epitopes contain the proline found at the amino end of the SM-3 determinant. These results are consistent with the idea that, in the cancer-associated mucin, premature termination of the carbohydrate side-chains results in the exposure of the SM-3 epitope.