Since one of the most frequent sites of human metastatic cancer is the liver, particularly in colon and rectum carcinoma, there is a special need for the development of an effective therapy. This study describes the parameters for reproducible production of poly lactic acid (PLA) microspheres with an average diameter of 37 microm and labelled with neutron-activated holmium-166 (Emax=1.84 MeV, t1/2=26. 8 h), suitable for use in internal radionuclide therapy of liver metastases. It is demonstrated that holmium-loaded PLA microspheres can be prepared by a relatively simple method, with incorporation of 17.0%+/-0.6% holmium (n=5), and that 20 GBq can be obtained from 400 mg neutron activatable microspheres. In order to produce this high amount of activity, the microspheres must be free of water and irradiation must be performed in a polyethylene vial, with a relatively low neutron flux (5x10(13) cm-2 s-1) within 1 h. Under these well-defined conditions minor surface changes were seen which barely affected total volume and consequently total radioactivity of the microspheres with a diameter of 20-50 microm. Overall structural integrity was maintained in terms of form and size. In vitro analyses showed that >99.3% of 166Ho activity was retained in the microspheres after 192 h incubation in PBS, plasma and leucocytes, while in liver homogenate retention was still 98.4%.