The response of synovial lymphocytes from a 65-year-old lady with persistent polyarthritis, to rubella antigen and a number of other microbial agents was studied over a period of 11 months by [3H]thymidine incorporation. The results were correlated with the ability to isolate rubella virus from both peripheral blood and synovial fluid during the same period. The patient showed initially a maximal stimulation index to rubella antigen assayed on five occasions over a five-month period. Rubella virus was detected in both peripheral blood and synovial fluid samples on three occasions during this period. Five months later the lymphoproliferative response of her synovial lymphocytes to rubella antigen had dropped to low levels, and virus could no longer be isolated from synovial exudates. At this time the patient's arthritis had become much less active, indicating that a good correlation existed between the presence of rubella virus, local lymphocyte sensitisation, and the inflammatory reaction.