Título en español.

1. Infection of the resistant tobacco variety Ambalema occurs at all ages. 2. Symptoms in resistant plants ordinarily consist of clearing of the veins followed by small chlorotic interveinal areas of a mild type. 3. Infection occurs and mild or severe symptoms appear in transplants when these are inoculated at time of pulling. These plants later recover in the field, but the virus is always contained in their tissues. 4. Infection of Ambalema plants was determined by inoculation of the juices extracted from those plants into N. glutinosa and Havana No. 38 tobacco. 5. Ambalema tobacco is also resistant to yellow tobacco mosaic, and celery mosaic. It should not be forgotten that symptoms are produced in inoculated plants but the effects produced by the disease are not very significant. 6. By adequate quantitative studies it was determined that the Ambalema tobacco is significantly more resistant to ordinary tobacco mosaic than Havana No. 38 tobacco at all stages of growth. 7. Ambalema tobacco is very susceptible to cucumber mosaic, yellow cucumber mosaic, potato ring-spot, Wingard's tobacco ringspot and spot-necrosis, and somewhat less susceptible to the mottle and veinbanding viruses. 8. The virus concentration was found to be lower in inoculated resistant plants nine weeks afte r inoculation (13 weeks old) than five weeks after inoculation (9 weeks old). 9. A mild form of mosaic was isolated from resistant plants inoculated with the ordinary tobacco-mosaic virus. In its properties this mosaic virus seems similar to the ordinary tobacco-mosaic virus. This may possibly be regarded as a case of attenuation.Resumen en español