To demonstrate an etiologic agent in rheumatoid arthritis we have cocultivated rheumatoid synovial cells with other cells and for the first time consistently observed viral particles by electron microscopy in these cocultivations. Inoculation of extracts of these preparations into suckling mouse brains caused severe pathology and death in the mice, and these effects could be serially passaged through additional mice. Our objectives are to confirm these observations, to attempt purification of the virus, and demonstrate its relevance to rheumatoid arthritis. We propose to investigate this organism further by attempting to develop antisera against purified preparations, to study previous infections in the general population, and to study the effects of the virus on cell cultures and on animals such as the rabbit or mouse.