The effects of whole spores from moulds were examined concerning histamine release using leukocyte suspensions from normal (non-atopic) individuals and grass pollen-allergic patients. Spores from Chaetomium globosum, Mucor racemosus and Aspergillus terreus caused no histamine release in concentrations up to 0.1 or 1 mg/ml, but they enhanced mediator release triggered by both IgE-dependent stimuli (grass-pollen allergen and anti-IgE antibody) and non-immunological stimuli (Staphylococcus aureus and calcium ionophore). Organic dust contains microfungi as well as bacteria and endotoxins. Earlier we have shown that bacteria release mediators from human lung cells and that bacteria and their endotoxins potentiate mediator release. It is now demonstrated that fungal spores also have potentiating ability. The content of all these microorganisms in dust may therefore be responsible for the symptoms observed after occupational exposure to organic dusts.