The evidence supporting organic dust as a cause of occupationally-related lung disease has substantially increased in the post two decades. The aim of this project was to test an assay which could be used to detect cytotoxic potential of organic dusts found in working environments. A cytotoxic assay with two cell lines was used: VERO and human A 549 carcinoma cells. Cells were exposed to extracts of dust samples collected from five different occupational environments with known adverse respiratory effects, using two different extraction protocols. Compared to water, alkaline soluble extract exerted the greater cytotoxic effect on the cells. With this assay it was possible to distinguish differences in the cytotoxic potential of the different types of organic dusts. The most aggressive dusts proved to be those with a high microbial content, such as compost and grain dust, which exerted an effect already after 2 hours of incubation at low concentrations.