The influence of temperature and humidity on the survival of airborne viruses was studied in a static system. Preliminary experiments with a bacteriophage showed that relative humidity was more important than temperature and absolute humidity. The effect of relative humidity was not dependent on the composition of the medium surrounding the virus particles. The survival of influenza and poliomyelitis virus are sharply influenced by relative humidity but in an opposite way. Influenza virus survives much better at lower humidities, poliomyelitis virus at higher humidities. In countries with moderate climates the period of increasing morbility for influenza, in winter, coincides with indoor conditions of relative humidity which are optimal to virus survival. For poliomyelitis the same is true during summer (Fig. 7). Indoor relative humidity is considered an important “seasonal factor” in the epidemiology of poliomyelitis and influenza and probably of other diseases.