SUMMARY: Experiments were made to determine whether the infectivity of virus suspensions was influenced by exposure to daylight of normal laboratory intensity or by exposure to artificial light. The infectivity of dilutions of virus suspensions held in bottles or cells was determined after exposure to light under a variety of conditions; the influence of exposure was indicated by comparison with the infectivity of control suspensions kept in darkness. The infectivity titres indicated by serial dilutions, in phosphate saline, of unfiltered fresh suspensions of egg strains of the viruses of vesicular stomatitis, influenza, Newcastle disease and fowl-plague following exposure to daylight for 4 hr. were 3-5 logarithmic units lower than those indicated by the dark controls. Changes of this magnitude suggest that with these virus strains significant errors might arise in infectivity titrations if suspensions were exposed to light for much shorter periods. Inactivation due to exposure to daylight was demonstrated in similar experiments with an egg strain of vaccinia virus, a mouse neurotropic strain of influenza virus and with guinea-pig strains and mouse neurotropic strains of the virus of vesicular stomatitis. Results of experiments with the virus of vesicular stomatitis indicated that inactivation by exposure to light may be eliminated almost completely by the inclusion of 10 % rabbit serum in the suspending medium. Losses of infectivity increased with the intensity of illumination and with the duration of exposure. Filtration or storage of certain initial materials also increased the susceptibility to inactivation of virus suspensions derived from them. The infectivity of suspensions of the virus of foot-and-mouth disease, even after filtration, was relatively stable during exposure to light.


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