Our objective was to determine if microspheres made from the biodegradable polymer poly(lactic acid) that contained rhenium could withstand the conditions of direct neutron activation necessary to produce therapeutic amounts of radioactive rhenium. The radiation damage of the polymer produced by gamma-doses of up to 1.05 MGy from Re-186 and Re-188 was examined by scanning electron microscopy and size exclusion chromatography. At a thermal neutron flux of 1.5 x 10(13)n/cm2/s the microspheres melted after 3 h in the nuclear reactor, but suffered little damage after 1 h of radiation and released less than 5% of the radioactivity during incubation in buffer at 37 degrees C. The radioactive microspheres produced in this manner have a specific activity too low for radioembolization for treatment of liver tumors, but could be injected directly into tumors or applied topically to the wound bed of partially resected tumors.