A monoclonal IgM antibody, H17, has been obtained from rats immunized with mouse fibrosarcoma cells from an in vitro established methylcholanthrene (MCA)-induced tumour. H17 shows specific and very selective binding to alpha-N-acetylgalactosamine (GalNAc alpha) when tested for reactivity to a panel of glycolipids. It cross-reacts with GalNAc alpha on the Forssman antigen extracted from dog small intestine, but not from the human blood group A determinant, a finding not commonly observed among antibodies with this specificity. Despite its specificity, H17 does not react with TA3-Ha, a mouse mammary adenocarcinoma, known to express the Tn antigen (GalNAc alpha-O-Ser/Thr). The uniqueness of H17 probably relates to the fact that it has been generated against an MCA-induced tumour rather than against the pure saccharide itself. Minimum energy conformation structures of different GalNAc alpha containing saccharide molecules were computer modelled to allow a plausible interpretation of the accessible site of GalNAc alpha for successful interaction with the H17 paratope as compared to other GalNAc alpha binding antibodies.