Impending new maximum contaminant levels (MCLs) for radionuclides, plus increased concern for radon in the air inside homes, have sparked new interest in these substances. An assessment of research needs, which also provided background information on completed and ongoing research projects, showed that Rn-222 represents the most serious threat to health of all the radionuclides in drinking water, leading to the anticipation that the new MCLs for these substances could be set at a relatively low level. Small public and private supplies were identified as being more vulnerable to radionuclide contamination than public systems serving more than a few hundred people, but more investigation is needed to assess the extent of the exposure and its associated risk. The previous belief that high levels of Rn-222 are confined to a few geographic areas was found to be false, largely because of a lack of a nationwide data base. Includes 93 references, tables, photos.