Thirty baboons were injected intravenously and intra-articularly with material from the joints of 19 patients with active rheumatoid arthritis or with control material. Fifteen of the 30 animals received synovial fluid cells or synovial membrane cells from 3 patients with seronegative arthritis. Ten animals received pooled cells from a total of 16 cases of seropositive arthritis. Five animals were given nonrheumatoid cells. No signs of arthritis were recognised in the 27 surviving animals during 3 years of observation. No significant biochemical, haematological, or serological changes occurred during this period, and no gross or microscopic evidence of synovial or systemic disease was found post mortem.