In order to develop reagents that can detect the exposed sialyl-Tn antigen (NeuAcα2,6GaINAcα 1-O-Ser/Thr) on tumour-associated mucins, we have prepared monoclonal antibodies (mabs 3C2 and 301, both IgM) against ovine submaxillary mucin (OSM; >98% of glycans as sialyl-Tn). These mabs showed strong reactivity with OSM and bovine submaxillary mucin (BSM; 50% of glycans as sialyl-Tn) but did not react with desialylated OSM or BSM. Sialic acid at I mg/ml did not significantly inhibit mab binding to OSM, suggesting that the linkage to GalNAc may be important for mab binding. 3C2 and 3D I also showed similar reactivity to sialyl-Tn reactive mab Bn.3, and detected Bn.3 capturedOSM in a sandwich ELISA. In Western blotting of mucus from a patient with a mucinous ovarian tumour, the mabs reacted with high molecular weight (>200 kDa) species. In immunohistochemistry, these mabs showed strong reactivity with most cancers of the colon, lung, and stomach, and also some tumours of the ovary and breast. There was only limited reactivity in normal tissue from these sites. The antibodies should be useful reagents for the detection of the sialyl-Tn antigen in human cancers.