New165Dy and166Ho macroaggregates(165Dy-MA,166Ho-MA) were prepared by reacting the aqueous solution of165Dy(NO3)3 and166Ho(NO3)3, respectively, with sodium borohydride solution in 0.2N NaOH.165Dy-MA and166Ho-MA are sterile suspension of macroaggregates consisted of Dy/Ho (50–56 wt.%) and boron (5–7%) in saline with mean particle size of 2.6 m (1–8 m range). Both MA could be prepared from the pre-made164Dy-MA and165Ho-MA followed by neutron irradiation. Even though the165Dy-MA and166Ho-MA suspension in saline were stored at 37°C for 24 hrs (Dy-MA), 10 days (Ho-MA) or autoclaved at 121°C for 30 minutes, there was no significant change in particle size and no leakage problem indicating the prepared165Dy-MA and166Ho-MA are sufficiently stable. The results of in-vivo retention studies using rabbits showed high retention (>99.5%) in knee joint even at 24 hrs (165Dy-MA) or at 10 days (166Ho-MA) after administration. Rabbits treated with intra-articular injections of164Dy-MA or165Ho-MA equivalent to 20–30 times the typical clinical dose showed no signs of any toxic effects at 1 month after administration. The ease with which the165Dy-MA and166Ho-MA can be made in the narrow size range and their high in-vitro and in-vivo stability make them attractive agents for radiation synovectomy.