Homogenates of synovium from patients with rheumatoid arthritis (RA) and osteoarthritis (OA) were centrifuged on caesium chloride density gradients to obtain isolates of a density similar to that of parvoviruses. Six of 11 RA isolates and none of six OA isolates reacted with an antiserum raised against a rheumatoid associated, parvovirus-like agent (RA-1 virus). An anti-B19 parvovirus antiserum did not react with any of the isolates tested. Electron microscopy of negatively stained preparations of the isolates showed that small particles of diameter 10 nm were abundant in most of the RA isolates (11/13) but absent from all OA isolates. Such particles, whose identification is unknown, were also present in RA-1 positive lysates prepared from cultured RA synovial cells. These results suggest that the RA-1 virus can be directly identified in RA synovial tissue and that the virus appears to be unrelated to the human B19 parvovirus.