Synovial fluid lymphocytes from patients with rheumatoid arthritis have been examined for evidence of a productive infection with retroviruses by electron microscopy, labelling with 3H-uredine, growth in soft agar, and culturing in conditioned medium. No such viruses were detected. In addition, the synovial lymphocytes were activated before fusion and cocultivation with several cell lines which have proved permissive for primate retroviruses. Monitoring these cultures subsequently by reverse transcriptase assay, labelling with 3H-uridine, and membrane immunofluorescence gave no indication that retroviruses were present.