Using the same experimental system which had been used to determine the pion relative biological effectiveness (RBE) for skin of mouse foot by Chaplin et al., we have determined the pion RBE for KHT sarcoma and SCCVII tumour transplanted in the foot of C3H mice. The pion RBE obtained by the tumour growth delay time method in comparison with 250 kVp X-rays at a dose rate of 150 cGy per min was determined to be 1.20,1.28 and 1.50 with single, 4 and’10 fractions, respectively, for KHT sarcoma. As for SCCVII tumour, it was determined to be 1.17, 1.45 and 2.05 with single, 4 and 10 fractions respectively. Therefore, it has been concluded that pions have various values of RBE depending on the tumour system involved. KHT sarcoma is a tumour which grows very rapidly, on the contrary, SCCVII tumour shows somewhat slower growth characteristics. These differences of RBE between these two tumour systems possibly stem from differences in the amount of hypoxic cells and/or their rates of reoxygenation. As for therapeutic gain factor, a maximum value of 1.45 was obtained with 10 fractions using the SCCVII tumour. Pions seem to be most effective (in multiple fractions) against tumours of relatively slow growt