Radiation-sensitive BALB/c MOPC 104E plasmacytoma was studied to investigate the effectiveness of total body irradiation in reducing tumor burden and preventing tumor regrowth. Also noted were the effects of such radiation on host immunity. BALB/c mice bearing 7-day-old occult tumors were not rendered free of disease when treated with radiation of 300 or 500 rad. However, all of the mice treated with 800 rad were cured. When mice bearing large tumors (24-days-old) were similarly treated, no cures were observed in the group treated with 300 rad. Of mice treated with 500 rad, 44% were cured, as were 100% given 800 rad of treatment. The percent reduction in serum MOPC 104E IgM increased with increasing radiation doses for the "large tumor" group. These observations show that total body irradiation may result in tumor elimination in an experimental animal multiple myeloma model. At lower radiation doses, host immunity appears to play an important role in achieving cure.