The 56 MAbs submitted to the ISOBM TD-4 Workshop were analysed by ELISA assay for reactivity against synthetic MUC1 mucin protein core-related peptides. With peptides comprising 5, 7 and 20 amino acids, 4/56 (7.1%), 33/56 (58.9%) and 51/56 (91.1%) of antibodies showed positive reactivity, respectively. Peptides with 60, 80, 100 and 120 amino acids showed 50/56 (98.2%), 54/56 (96.4%), 53/56 (94.6%) and 47/56 (83.9%) antibodies with positive reactivity, respectively. The reactivity of each MAb with synthetic peptides was classified into 3 groups. In group A (15 MAbs), the reactivity increased depending on the peptides in length and was maximal with 20-, 60-, 80-, 100- and 120-mer peptides. In group B (24 MAbs), most antibodies showed very weak binding affinity to peptides with a small increase in reactivity with the 100- and 120-mer peptides. In group C (17 MAbs), the reactivity showed strong binding affinity with the 20-mer peptide followed by a reduced uneven pattern of reactivity with the 60-, 80-, 100- and 120-mer peptides. These results show that the binding affinity of MAbs against synthetic peptides does not depend on the recognition of antibodies to the epitopes of core protein.