Derangements of synovial membranes and cartilage occur early in the course of rheumatoid arthritis. These important alterations of the joint tissues are probably the in vivo reflections of complicated inflammatory and immunological events. In our laboratory we have been interested in studying alterations of synovial lining cells in rheumatoid arthritis, most recently by the use of serially propagated cultures of these cells. The cellular traits described in such cultures serve to distinguish these synovial cells from other types of human fibroblasts, and several cellular alterations have been found in cultures derived from membranes of rheumatoid arthritic patients. One important finding is increased resistance of cultured rheumatoid cells to infection with rubella and NDV; this and other cellular changes suggest the possibility of an occult virus infection in the rheumatoid cells. Such viral persistence could be theoretically linked with the immunologic aberrations in rheumatoid arthritis, discussed in this symposium.