In this study several air samplers have been compared in more than one environment, in contrast to most environmental studies of airborne microorganisms in which only one type of sampler has usually been used. Very few comparative studies have been made on the efficiency of different apparatus and methods of cultivation, and therefore in most cases published results are difficult to compare and evaluate. In this study we have compared some samplers in common use, and found that many different factors must be taken into account when sampling viable microorganisms. The results will depend on type of apparatus, the environment, the conditions of cultivation, and the number of measurements. The efficiency ratio of an Andersen and a cyclone sampler varied depending on which type of environment was sampled.