A panel of 56 murine monoclonal antibodies submitted to the ISOBM TD-4 (MUC1) Workshop was analysed for reactivity against nonglycosylated and in vitro GalNAc glycosylated peptides. Twenty-six antibodies reacted with nonglycosylated MUC1 peptides containing 3-5 tandem repeats. The reactivities of most of the antibodies were not affected by GalNAc glycosylation of the peptides. Antibody #147 (decoded as BCP9) reactivity was inhibited when 3 mol of GalNAc per repeat were incorporated in the peptide (at sites T in GVTSA and ST in GSTAP), whereas the reactivity with GalNAc glycosylated peptides with 2 mol of GalNAc per repeat (sites T in GVTSA and T in GSTAP) was unaffected. This is in agreement with the epitope defined as GSTAP.