[en] Chronic exposure to beta radiation has been tested as a tumor promoting or progressing agent. The dorsal skins of groups of 25 female SENCAR mice were chemically initiated with a single exposure to DMBA, and chronic exposure to strontium-90/yttrium-90 beta radiation was tested as a stage 1, stage 2 or complete skin tumor promoter. Exposure of initiated mice to 0.5 gray twice a week for 13 weeks produced no papillomas, indicating no action as a complete promoter. Another similar group of animals was chemically promoted through stage 1 (with TPA) followed by 0.5 gray of beta radiation twice a week for 13 weeks. Again no papillomas developed indicating no action of chronic radiation as a stage 2 tumor promoter. The same radiation exposure protocol in another DMBA initiated group receiving both stage 1 and 2 chemical promotion resulted in a decrease in papilloma frequency, compared to the control group receiving no beta irradiation, indicating a tumor preventing effect of radiation at stage 2 promotion, probably by killing initiated cells. Chronic beta radiation was tested three different ways as a stage 1 tumor promoter. When compared to the appropriate control, beta radiation given after initiation as a stage 1 promoter (0.5 gray twice a week for 13 weeks), after initiation and along with a known stage 1 chemical promoter (1.0 gray twice a week for 2 weeks), or prior to initiation as a stage 1 promoter (0.5 gray twice a week for 4 weeks), each time showed a weak (∼ 15% stimulation) but statistically significant (p<0.01) ability to act as a stage 1 promoter. When tested as a tumor progressing agent delivered to pre-existing papillomas, beta radiation (0.5 gray twice a week for 13 weeks) increased carcinoma frequency from 0.52 to 0.68 carcinoma/animal, but this increase was not statistically significant at the 95% confidence level. We conclude that in the addition to the known initiating, progressing and complete carcinogenic action of acute exposures to ionizing radiation, chronic exposure to beta radiation can act as a weak stage 1 tumor promoter of chemically initiated skin cells