The successful use of radioactive colloidal gold (Au198) in the treatment of malignant peritoneal and pleural effusions prompted its trial use in cases of persistent synovial effusion in which orthodox methods had consistently failed. A trial was carried out on 25 joints in 22 patients with persistent effusions. The injections were given intra-articularly. Follow-up ranging from ten months to six years showed three failures. Pain was almost always relieved and movement facilitated. No complications, either immediate or delayed, occurred as a result of the injection. The 10-mc dose used was estimated on an assumption of surface area of the synovium. The clinical success of the method would suggest that this dose is adequate to relieve effusion.