Mucins are large highly glycosylated molecules that have been postulated to interfere with certain cell–cell interactions. Steric, charge and specific signalling effects have been postulated for the inhibition by cell-surface mucin molecules. In this report we evaluate the inhibitory effects of bovine submaxillary mucin (BSM), a mucin without specific lymphocyte interactions, on lymphocyte function. BSM inhibits the adhesion of lymphocytes when coimmobilized with intercellular adhesion molecule-1 (ICAM-1) and blocks the activation of T lymphocytes when coimmobilized with anti-CD3. These data demonstrate a general mucin effect on lymphocyte adhesion and activation that is primarily steric in nature and implicates mucins as general barriers to lymphocyte–tumour cell interactions. Mucin blockade of cell–cell interactions may explain why mucinous tumours are often associated with a poor prognosis.