Farmers and farm workers can be exposed to large concentrations of airborne fungi and bacteria including actinomycetes, and microbial constituents such as endotoxins. Measurement methods for microorganisms may give different results andneed to be further developed to allow personal sampling and species characterization o fviable and non-viable microorganisms. Farm work includes many differently exposed tasks and processes. A large number of measurements is therefore needed to estimate long-term exposure in epidemiological studies of farming populations. A more efficient strategy is probably exposure modeling using different determinants of exposure. The literature was therefore reviewed for studies on determinants of exposure to microorganisms in agriculture. In most studies tasks, process and/or production had been studied. Little information is available on other determinants. It was found that exposure to fungi was high during handling grain, hay and bedding material, especially when mouldy, and when tending cattle; exposure to bacteria was high during handling of grain, hay and bedding material and in animal houses; and exposure to endotoxins was high during chopping of bedding material and in animal houses except probably in cowsheds. Exposure showed wide variability between measurements of the same task even within studies. Further studies of other determinants of exposure are recommended in order to improve the accuracy of exposure assessment in epidemiological studies and in future compliance testing.