The infection of selected lymphocytes from a rheumatoid arthritis patient with Epstein-Barr virus resulted in an immortalized cell line that secretes a monoclonal rheumatoid factor (RF). The cloned line has been growing for more than 24 months, and constantly produces a monoclonal IgM, lambda, 19S, RF (1-2 micrograms/ml/106 cells). The RF agglutinates human and rabbit IgG (but not IgM) and also protein A-coated erythrocytes, but fails to do so to mouse, goat and swine IgG-coated erythrocytes. When bound to immune complexes, this monoclonal RF dose not bind complement. In the cell supernatant RF is the only immunoglobulin and it comprises approximately 5% of the total proteins. The affinity of RF to aggregated human IgG, as detected in inhibition experiments, is higher than that of Fc receptors found on human non-T lymphocytes, K562 and Daudi cell lines.