The viability of non-sporing bacteria in a dry state was examined. The number of viable cells was determined at various time intervals, after inoculation onto cotton lint and a glass plate. Viable cells of Mycobacterium bovis were detected more than 2 months after inoculation, and this was the most resistant species to dry conditions among the bacteria tested. In the case of the Gram-positive cocci tested and one species of Gram-negative rods, Acinetobacter calcoaceticus var. anitratus, the viable cell numbers decreased by only 2 log10 by 25 days after inoculation, indicating that they are highly resistant to dry conditions. In contrast, Gram-negative rods other than A. calcoaceticus lost their viability very rapidly and no viable cells were detected 7 h after inoculation. In the presence of proteins such as bovine serum albumin and horse serum, the viability of bacteria in the dry state increased. This protein effect on bacterial survival was marked for Escherichia coli and Salmonella typhimurium but not for Pseudomonas cepacia and Xanthomonas maltophilia. These results indicate why Gram-positive cocci and A. calocoaceticus are capable of causing airborne infections. It is also suggested that enterobacteria such as E. coli could cause infections when they survive in a dry environment along with proteins derived from body fluids.