(1-->3)-beta-D-glucan is a polyglucose structure in the cell wall of moulds, some bacteria and plants. Due to its unique (1-->3)-beta linkage it binds to specific receptors on phagocytosing cells and induces changes in their metabolism. Under realistic environmental concentrations, available data suggest that these changes express themselves as alterations of the defense mechanisms to other agents. Inhalation of (1-->3)-beta-D-glucan in humans causes symptoms from the upper respiratory tract and induction of cytokines in blood monocytes. (1-->3)-beta-D-glucan can be used as a marker of mould biomass in field studies. Relationships between the amount of (1-->3)-beta-D-glucan and the extent of symptoms as well as lung function changes and inflammatory markers have been described. In view of the mechanisms involved in the normal development of the immune system, children seem to be a particular group at risk due to (1-->3)-beta-D-glucan exposure.