The growth rate, as measured by increases in volume of a transplanted tumor (adenocarcinoma), was compared in mice that were irradiated locally to the head and in nonirradiated mice. 6-month-old female C3H/HeJ mice were used, and those that were irradiated received dosages of 100 and 200 r from a Keleket 220 KVP 20 Ma X-ray machine. The dosages were delivered at 6.66 r/sec through a 0.25 cu/1.0 aluminium filter. The beam from the X-ray tube was then collimated through a series of lead shields before reaching the cranium of the mouse. 24 h after irradiation, some mice were transplanted into the left side of the axillary fold, with small pieces, approximately 2 mm3, of tumor tissue. Careful weekly examinations for 15 weeks after the initial transplantation revealed major differences in tumor volume between irradiated and nonirradiated animals.