Exposure to high airborne levels of noninfectious microorganisms is recognized as a cause of respiratory symptoms and disease among workers handling biological materials, such as farmers, sawmill workers, and workers handling municipal waste and fuel chips. Risk assessment is difficult because occupational exposure limits for noninfectious microorganisms have not been established. Many different methods are used for the measurement of airborne microorganisms, which are based on impaction, impingement, or filtration. Samples can be analyzed by methods that are culture-based or nonculture-based and that may estimate different microbial entities: culturable microorganisms by culture-based methods, microbial cells by microscopic methods, and microbial constituents and products by chemical, biochemical and immunochemical methods. Sources of errors and validation studies of these methods are reviewed and methods are evaluated for exposure assessment in epidemiological studies and for future compliance testing. At present it is not clear which microbial bioaerosol components should be assessed. Culture-based methods are probably not satisfactory because nonviable microorganisms and microbial constituents and products also may cause health effects. Culture-based methods are poor surrogates for nonculture-based methods and have poor precision. However, identification of microorganisms is most readily performed by culture-based methods. Filter sampling is preferred for personal exposure measurements because filters can be analyzed by a variety of nonculture-based methods, and filter sampling may be adapted to recently adopted criteria for health-related size fractions.