Vast numbers of spores of the thermophilic actinomycetes Excellospora flexuosa, Thermomonospora alba, T. curvata and T. fusca were collected from the air in fermentation tunnels during the spawning of mushroom compost, i.e. over 10(9) CFU m-3 of air. Five different genera of fungi, namely, Aspergillus, Aureobasidium, Cladosporium, Penicillium and Scytalidium, were found at only 10(3) CFU m-3 of air. Agaricus bisporus, used for spawning, was absent. Sera of 10 mushroom growers affected by Mushroom Worker's Lung (MWL) were tested by a qualitative dot-ELISA for antibodies against the spores of these micro-organisms. All 10 were positive for one or more of the actinomycetes E. flexuosa, T. alba, T. curvata and T. fusca. No antibodies were found against Streptomyces thermovulgaris, Thermoactinomyces vulgaris and T. sacchari nor against the fungi Aspergillus fumigatus, Penicillium brevicompactum, P. chrysogenum, Scytalidium thermophilum and Trichoderma viride. Sera of 11 of 14 workers engaged in routine spawning of compost in tunnels reacted positively with 1 or more of the actinomycetes. Their 10log serum titres increased with the duration of employment to an upper limit of 2.5. The sera of 19 non-exposed individuals were negative. Because high numbers of spores of E. flexuosa, T. alba, T. curvata and T. fusca were present in the air that was used for successful inhalation provocation of mushroom workers with MWL and because of the elevated serum titres of these workers, we presume these organisms to contribute in the occurrence of MWL.