We developed the 166Ho-chitosan, the new radiation synovectomy agent. Holmium is the more practical isotope based on its longer half-life. And chitosan, is ideal and suitable particles based on its soluble and biodegradable characteristics. We investigated the biocompatibility of the 166Ho-chitosan complex to evaluate the suitability as a radiation synovectomy agent. In vitro stability test, the 166Ho-chitosan complex suspension in saline was stored at room temperture and 37degrees C for 25 days and decay rate was of determined by ITLC(Instant Thin Layer Chromatography). In vivo stability test, the 166Ho-chitosan complex was injected into rabbit joints and followed by gamma camera imaging to quantify extra-articular leakage. Biodistribution study, the 166Ho-chitosan complex was injected into rabbit joints. After 48 hours heart, liver, urinary bladder, spleen, lung, brain, kidney, blood were extracted and radioactivities were measured. In vitro stability test, there was no significant change of radioactivity and no leakage problem indicating the prepared 166Ho-chitosan complex is sufficiently stable. In vivo stability tests revealed that more than 98% of the 166Ho-chitosan remained in joint over a 2 days period. The mean retention percentage of 166Ho-chitosan in knee were 99.9%, 99.9%, 99.8%, 99.7% at 2 h, 6 h, 1 day and 2 days, respectively. A biodistribution study of the rabbits revealed that leakage to heart, liver, urinary bladder, spleen, lung, brain, kidney, blood is 0.71, 1.5, 0.50, 1.5, 0.25, 0.26, 0.81, 0.065(% Injected Dose x 10-3/gram), respectively. The 166Ho-chitosan complex shows less leakge than any other radiation synovectomy agents. Our results indicate that 166Ho-chitosan have the biocompatibility and the suitability as a radiation synovectomy agent.