The skin of 50 pigs has been irradiated with negative pi mesons and with X rays in order to determine the RBE for early epidermal and later dermal damage. Late fibrosis was not studied. Four, 7, 9 and 10 fractions were used. An estimate of the RBE was made from the reactions on each pig for both early and late damage so that interanimal variability would be avoided. The data were also averaged to obtain mean dose response curves. There was no tendency for higher RBE's for late than for early skin damage. These pig studies have demonstrated an RBE of about 1.5 for early epidermal reactions and a slightly lower RBE (approximately 1.4) for later dermal damage in the same animals. This indicates that at doses of about 2.0 to 3.5 Gy pions, the medium wave skin damage is unlikely to be more severe than would be predicted from the early skin reactions and the accumulated clinical experience with X rays. However, if the trend to a steeper slope for the RBE versus dose per fraction for late injury is correct, as indicated by other published studies a relative increase in the late injury might be expected if much lower doses per fraction are used. The present clinical studies at Vancouver using 15 X 2.1 Gy pions indicate that an RBE of 1.5 is appropriate for epithelia, brain and colorectum.