The sampling efficiencies of the Biotest centrifugal sampler, the SAS surface air sampler, the Andersen two stage sampler, and the Andersen single stage sampler with the membrane filter sampler were compared for use in assessing potential exposures to microorganisms in biotechnology based production facilities. Each sampler was challenged with a microbial surrogate aerosol under controlled conditions in a horizontal chamber designed for the study of aerosol samplers. The findings indicated that the Andersen two stage sampler differed from the other samplers except for the reference sampler, the filter cassette, in terms of the level of measurement on the average and by particle size level. The Andersen single stage sampler under sampled the reference sampler by -47%. The Biotest and the SAS sampler both under sampled the reference sampler by -81% and -57%, respectively. There was not a statistical difference among particle size ranges for the bias of the Biotest sampler. However, there was a statistical difference among particle size ranges for the bias of the SAS sampler.