The inactivation of Pigeon Pox and Rous sarcoma viruses (R.S.V.) at various relative humidities (R.H.), with and withouti-inositol, was studied. Pigeon Pox virus was found to be stable in aerosols and was little affected by changes in R.H. R.S.V., however, was extremely sensitive to R.H. and survived best at R.H. levels above 70%. Inositol at a concentration of 6.0% was able to prevent the inactivation of R.S.V. Holding the viruses in aerosols at 80% R.H. or protected with inositol at 30% R.H. appeared to increase their virulence towards the assay eggs and experimental animals.