A total of 316 bacterial strains, including psychrophiles, mesophiles and thermophiles, were isolated and identified from indoor dusts in schools, children's day care centres and animal sheds. Several species which had not previously been reported from indoor environments were found: Sphingomonas, Brevibacterium, Nocardiopsis, Deinococcus and Rhodococcus/Gordona. A new psychrophilic actinomycete genus was also found in animal sheds, representing a new undescribed peptidoglycan type and an unusual whole-cell fatty acid composition. The indoor dusts of animal sheds contained mainly the Gram-negative genera Pseudomonas, Pantoea, Flavobacterium and Xanthomonas early in the indoor feeding season, but changed to a composition dominated by Bacillus, Micrococcus and mesophilic and thermophilic actinomycetes towards the end of the season. The dust contained, and air-borne bacterial flora in schools and day care centres were dominated by, Gram-positive bacilli and actinomycetes, notably Bacillus cereus, Brevibacillus brevis, B. licheniformis, B. subtilis and species of Arthrobacter, Corynebacterium, Rhodococcus/Gordona, Nocardiopsis sp., Deinococcus, Staphylococcus and Micrococcus. Indoor air and dust contained Klebsiella oxytoca, Acinetobacter calcoaceticus, Ac. lwoffi, Bacillus cereus and Nocardiopsis dassonvillei with the status of hazard group II. Indoor dusts of animal sheds contained eight different 3-hydroxy fatty acids, the 2-hydroxy fatty acid 14: 0 and two 10-methyl fatty acids, whereas in dusts from schools and day care centres, these were below the detection level (< 3·5 ng mg−1). The 3- and 2-hydroxy fatty acids could be assigned to one or more of the dust-contained cultivable strains, but 10-methyl C16: 0 was not present in any of the strains isolated. The dusts from schools and children's day care centres contained 0·2–0·3 ng of endotoxin mg−1 and 0·5–1·4 ng of β-d- glucan mg−1, whereas the dusts from animal sheds contained more 0·3–41 ng mg−1 and 8–35 ng mg−1, respectively.