Erbium-169 citrate is anti-inflammatory and antirheumatic radiopharmaceutical agent. Erbium-169 decays with half-life 9.4 days by the emission of beta particle and very low energy gamma radiation. The average radiation penetration is 0.3 mm in the soft tissues and 0.2 mm in the cartilage. Colloid Er-169 is used in radiosynovectomy on rheumatoid arthritis patients whose condition is resistant to standard methods of treatment. During this procedure, colloid erbium-169 citrate is directly injected into the synovial cavity of the affected joint. The data indicate that after intra-articular injection, radioactive colloid particles are picked up by superficial synovial cells. Under the effect of irradiation, necrosis of the superficial synovial layer takes place, followed by phagocytosis of the necrotic tissue and delineated inflammatory flare. This temporarily halts synovitis and improves synovial joint function.