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Studies in aerobiology are often conducted on a comparative basis; that is, the numbers of viable airborne organisms obtained under one set of circumstances are compared to those obtained under other conditions. For samplers employed in such studies, reproducibility and ease of handling often are considered more important than absolute efficiency because the final conclusions are derived from internal comparisons between various data collected using the same samplers throughout. In planning studies at Georgia Institute of Technology on the effect of chemical vapors on the viability of airborne bacteria, the critical-orifice liquid impinger was selected because of the demonstrated reproducibility and ease of handling (Rosebury 1947; Henderson 1952). Subsequent experience justified this choice, although it was found that the efficiency was significantly affected by the composition of the liquid used in the sampler. Further studies showed that this effect was responsible for potentially large errors when estimating particle size by the method of comparing aerial concentration to number settling per unit area per time unit. This experience indicated the need for consideration of other factors which might influence the operation of critical-orifice liquid impingers when used as samplers for bacterial aerosols. MATERIALS AND METHODS The test culture. The investigations were carried out with carefully differentiated cultures of Serratia marcescens strain ATCC 274, maintained in 0.3 per cent beef extract broth. The cultures were 45 to 48 hr old when used in tests (Kethley et al., 1956). Chambers and equipment for the production and maintenance of the bacterial aerosol. The organisms were

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