Radioisotopes have been employed in the therapy of chronic arthritis, in particular, rheumatoid arthritis for many years. A variety of isotopes have been popularized, and in the last ten years a colloidal solution of radioactive chromic phosphate 32P has been in use apparently with equivalent efficacy to others such as 169erbium, 90yttrium, and 165dysprosium. No controlled studies on this modality have been reported and few animal studies were found. The efficacy of therapeutic doses of 32P as a medical synovectomy and its effect on rabbit joints with antigen-induced arthritis were observed in 62 arthritic knee joints in 31 adult rabbits treated on one side with 0.1 microCi of 32P, the opposite serving as control. The animals were observed over a period of 11 months and examined by histologic and biochemical means. The synovium showed no evidence of radiation necrosis in treated joints. Cartilage of treated and control joints showed similar changes consistent with chronic arthritis, persistent synovitis, progressive chondrocyte degeneration, and decreased matrix metachromasia. The radiosynovectomy had neither removed synovium nor protected the cartilage. Its efficacy in humans is therefore questionable.