Human respiratory mucins are secreted by goblet cells and mucous glands of the respiratory mucosa. They consist of a broad family of complex glycoproteins with different peptides, or apomucins, corresponding to several genes located on at least three different chromosomes. Glycosylation, the major posttranslational phenomenon, is responsible for about 70-80% of the weight of mucins: it produces an extraordinary diversity of O-glycosidically linked carbohydrate chains which are expressed as several hundreds of different chains in the mucins of a single individual. The variety of mucin peptides and the diversity of carbohydrate chains probably allows many interactions, especially with microorganisms: this may be an essential factor in the defence of the underlying respiratory mucosa.