Aiming to identify factors causing the adverse health effects associated with moisture damaged indoor environments, we analyzed immunotoxicological potential of settled dust from moisture damaged and reference schools in relation to their microbiological composition. Mouse RAW264.7 macrophages were exposed to settled dust samples (n= 25) collected from moisture damaged and reference schools in Spain, The Netherlands and Finland. After exposure, we analyzed production of inflammatory markers [nitric oxide (NO), tumor necrosis factor (TNF)α, interleukin (IL)-6 and macrophage inflammatory protein (MIP)2] as well as mitochondrial activity, viability, apoptosis and cell cycle arrest. Furthermore, particle counts, concentration of selected microbial groups as well as chemical markers ergosterol, 3-hydroxy fatty acids, muramic acid, endotoxins and glucans were measured as markers of exposure. Dust from moisture damaged schools in Spain and The Netherlands induced stronger immunotoxicological responses compared to samples from reference schools; the responses to Finnish samples were generally lower with no difference between the schools. In multivariate analysis, IL-6 and apoptosis responses were most strongly associated with moisture status of the school. The measured responses correlated with several microbial markers and numbers of particles, but the most important predictor of the immunotoxicological potential of settled dust was muramic acid concentration, a marker of Gram positive bacteria. This article is protected by copyright. All rights reserved.