Chitosan is a polysaccharide of natural orgin obtained by full or partial deacetylation of chitin, a very abudant natural polymer, which has the properties of biocompatibilities, bioaffinities, and biodegradabilities. The free amino group of chitosan should be participated in forming chelate with holmium (β-emitter). Ho-166(NO3)3. 5H2O of high radionuclidic purity of upto 99.9% was made by neutron irradiation of naturally occuring Ho-165(NO3)3., 5H2O, and then reacted with the prepared chitosan solution. The effect of pH, reaction time, the concentration and viscosity of chitosan and the amount of Ho-166 on forming Ho-166-chitosan complex (Ho-166-CHICO) were investigated. Ho-166-chitosan macroaggregate(Ho-166-CHIMA) was made from Ho-166-CHICO. Their physical properties such as radionuclidic purity, particle size distribution, stability in vitro and vivo were examined. Their high in vitro and vive stability makes them attractive agents for interna1 radiotherapy by local administeration.