Epidemiological studies have shown that the inhabitants of indoor environments with moisture damage are subjected to an in-creased health risk. The ascertained complaints are probably caused by microorganisms which have developed in the affected material. Apart from moulds, which have been sufficiently studied in this context, a large number of bacteria are also found, their exact differentiation is, however, still insufficient due either to the lack or complexity of analytical methods. The bacteria group of actinomycetes today systematically grouped as filamentous, grampositive bacteria within the actinomycetales is of particular interest in this connection, since among them there are pathogens, causative agents of allergic symptoms, as well as many species which can produce secondary metabolites that are irritating or even toxic for humans. The most important allergic disease caused by actinomycetes spores is the allergic alveolitis, which occurs mainly in microbiologically highly polluted working areas. Potent allergens are conidia from Micropolyspora faeni, Thermoactinomyces vulgaris, Thermoactinomyces sacchari and Saccharomonospora viridis. Further species which cause allergic reactions are Streptomyces album and S. olivaceus. Among the species which can cause infections and live on moist building materials Nocardia are of particular interest. These species can lead to Nocardioses in patients with a weakened immune system. Some species are able to produce metabolites with antibacterial activity. Some metabolites function as enzyme inhibitors or immunomodulators, while others are used in tumor therapy. Besides, some of the metabolites can be useful while others are toxic. Toxins of Steptomyces spp. have already been detected in the air and dust of moist indoor environment. The differentiation of unknown Actinomycetes species often requires a combination of morphological, chemical (e.g. fatty acid analysis) and molecular-biological methods (e.g. DNA analysis). While the numerical registration of Actinomycetes colonies is possible as a matter of routine, the differentiation of species can only be carried out by specialists due to the insufficiency of morphological differences and/or insufficient fatty acid or DNA sequence data banks. Even among those Actinomycetes which are often registered in areas of indoor moisture damage, unknown species have been found. Although our studies are still in the initial stages, it can be seen that the appearance and distribution of Actinomycetes depend on the material, the humidity and the age of damage. On the basis of the results obtained so far, background concentrations of living Actinomycetes in unpolluted indoor environments are generally under 1,000 CFU/g material and under 50 CFU/m(3) air. In those cases where Actinomycetes were found in the air, they often belonged to the spore-producing genera Streptomyces, Nocardia, Nocardiopsis or Pseudonocardia.