Irradiation of 20mg of natural Dy(NO3)3 in a neutron flux of 2*10(13)n/cm2 sec for 4 hours gave 5.76 Ci of 165Dy(specific activity, 610mCi/mg Dy) with high radionuclidic purity (>99.9%). 165Dy-MA was prepared in a quantitative yield by reacting the aqueous solution of 165Dy(NO3)3 with sodium borohydride solution in 0.2N NaOH. Coulter particle analyzer exhibited mean particle size of 2.6um (range 1~6um). Even though the 165Dy-MA suspension in saline was stored at 37`C for 24hours of autoclaved at 121`C for 30minytes, there was no significant change in particle size and leakage problem indicating the prepared 165Dy-MA is sufficiently stable. In-vivo retention studies were carried out by administering 165Dy-MA into the knee joing space of normal rabbits. Gamma camera analysis showed high retention in joing space of normal rabbits. Gamma camera analysis showed high retention in joing space even at 24 hours after administration (>99.9%) The ease with which the 65Dy-MA can be made in the narow size range and their high in vitro and vivo stability make them attractive agents for radiation synovectomy.