Microbial content in dusts such as bacteria, endotoxins and fungal spores are thought to be important causative agents for the symptoms in organic dust‐related diseases. Micro‐organism‐induced mediator release was therefore examined in human cells. Bacteria were found to trigger the release of histamine and leurotriene B4 from bronchoalveolar cells, and in suspensions of dispersed lung and tonsillar cells they induce the release of histamine and prostaglandin D2. Basophil histamine release was triggered by both bacteria and their endotxins. Furthermore, histamine release caused by allergic as well as non‐allergic reactions was enhanced by bacteria, endotoxins and fungal spores of mould. These effects of dust components may be crucial for the symptoms in q a n i c dust‐related diseases, since the mediators are of key importance to the broncho‐obstructive and inflammatory events in these disorders.