EMT-6 mouse tumors were treated with (a) 1 or 5 fractions of either x rays or neutrons or (b) a mixture of both in which 2 fractions of neutrons (n) plus 3 fraction of x rays (x) were given in 5 days in the sequence n-n-x-x-x or n-x-x-x-n. Using local tumor control as an end point, neutron RBEs of 1.7 for single fractions and 2.6 for 5 fractions were obtained. The two mixed schemes yielded RBEs of 1.5 and 1.4, respectively. Therapeutic gain factors, calculated as the ratio of tumor to skin RBE with neutrons or mixed radiations, were highest for the mixed fractionation schemes. These results are due to an apparent enhancement of the neutron effect in the tumor but not in the skin with these regimens. Other normal tissues must be irradiated with mixed schemes to determine whether this phenomenon is limited to the skin.