X-irradiation enhanced a new blood vessel formation occurring in the course of a local GvH reaction in mice. Maximal enhancing effect was exerted by single total body irradiation doses ranging from 700 to 1050 R. Fractionation in daily doses smaller than 200 R did not exhibit enhancing effect. Proliferative ability of angiogenesis inducing cells is not necessary for enhancing effect of irradiation.