Monoclonal anti-mucine antibodies BC1, BC2 and BC3 produced using human milk fat globule membrane react with a synthetic peptide p1-24 (PDTRPAPGSTAPPAHGVTSAPDTR) representing the repeating amino acid sequence of the mucin core protein. The minimum epitope recognized by these three monoclonal antibodies (mAb) in p1-24 was contained in the five amino acids APDTR. To analyse the variation of position of the epitope, various modifications of the APDTR sequence were made by synthesizing peptides and testing by direct binding and inhibition enzyme-linked immunosorbent assays. Firstly, peptides p13-32 and C-p13-32, in which the epitope APDTR was placed in the middle instead of the C-terminal as in p1-24, were examined. These peptides had a greater reaction with mAb BC1, BC2 and BC3 compared with the reaction with p1-24. Secondly, A-p1-24 and TSA-p1-24 were made wherein two APDTR epitopes were present--these peptides were shown to bind two IgG antibody molecules. Finally, the contribution of each amino acid in the APDTR epitope was studied using the pepscan polyethylene rods, making all 20 of the amino acid substitutions in each position for SAPDTR (the minimum epitope APDTR with an adjacent amino acid S). In the 120 peptides examined there were some 'permissible' substitutions in A, D and T but not in P or R for BC1 and BC2; there were more 'permissible' substitutions for BC3; different substitution patterns were found with each antibody and some substitutions gave an increased reaction compared with the native peptide SAPDTR. The studies are of value in analysing the reaction of antibodies with epitopes expressed in breast cancer and in determining the antigenicity of synthetic peptides.