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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 α-N-acetytgalactosamine (GalNAcα) when tested for reactivity to a panel of glycolipids. It cross-reacts with GalNAcα 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 lo express the Tn antigen (GalNAcα-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 GalNAcα containing saccharide molecules were computer modelled to allow a plausible interpretation of the accessible site of GalNAcα for successful interaction with the H17 paratope as compared to other GalNAcα binding antibodies. [ABSTRACT FROM AUTHOR]