Abstract In ninety Syrian hamsters, divided into six groups, twenty animals were irradiated with 1,530 r of cobalt-60 x-rays and twenty animals were irradiated with 1,530 r of 200 KVP x-rays prior to three weekly applications of DMBA to the right buccal pouches. The radiation was applied as a single continuous dose limited to the area of the right cheek and jaws. Control groups for the DMBA application and the two modalities of radiation were studied. Cobalt-60-irradiated hamsters developed carcinomas at a rate similar to the control animals. Animals irradiated with 200 KVP developed carcinomas later than their controls. This retarded carcinogenesis in the orthovoltage group was interpreted in terms of the DMBA acting upon tissue, with degenerative alterations induced by the 200 KVP radiation, that is, the radiobiologic effect resulted in a slowing of the chemobiologic effect.