An RBE for acute skin reaction to peak pions, for a specific fractionation scheme, has been established at 1.40-1.44. The time of development of acute skin reactions varied in 2 patients with 30 metastatic skin nodules treated with doses varying from 1,175 to 1,951 peak pions and 2,350 to 3,901 rads of 100 kVp x rays. The reactions reached their peak approximately one week apart at all dose levels with both forms of radiation. No untoward effects have been seen in epidermis or subcutaneous tissues as late as 169 days after the start of treatment. All nodules disappeared and have not returned 169 days after treatment. There is a strong suggestion that the response of thick tumors is sensitive to the proportion of high LET radiation deposited at various levels in the tumor.