The sialosyl-Tn (STn) antigen is a mucin-associated carbohydrate antigen expressed by a variety of adenocarcinomas. In the colon, expression of this antigen has been associated with a poor prognosis, independent of tumor stage or histology. The present study was performed to determine whether this adverse clinical outcome might be due to an interaction between STn-positive mucin and natural killer (NK) cell cytotoxicity. Ovine submaxillary mucin (OSM), a mucin highly rich in STn antigen, partially inhibited NK cell cytotoxicity against K562 target cells, but only at high concentrations. Low concentrations of OSM were not inhibitory but became markedly inhibitory in the presence of ammonium ions. Two other STn-positive submaxillary mucins also markedly inhibited NK cytotoxicity when combined with ammonium ions. Removal of sialic acid from OSM reversed the OSM/ammonium-mediated inhibition of NK cell activity. Unlike the submaxillary mucins, two mucins derived from human breast and lung cancer cells which lack the STn antigen, did not inhibit NK cell activity in this system. Likewise, four other non-mucin glycoproteins which lack STn expression did not inhibit NK cells despite having levels of sialic acid that were, in some cases, comparable to submaxillary mucin. These results indicate that mucins bearing the cancer-associated STn antigen can effectively inhibit NK cell cytotoxicity in the presence of ammonium ions. While this NK cell inhibition is likely to be caused by ammonium, mucin markedly enhances this effect, thereby implicating a novel immunomodulatory property of mucin.