Biological elfect of the ,192,Ir high activity source on LA,795, tumor of mice and HCT-8 cells have been investigated when decay of the source power from 340.4 GBq to 81.4 GBq No marked difference was found between the two cell survival curves of HCT-8 cells and both of them compared with that of the X-ray irradiation the value of relative biological effect (0.1 survival) was 0.43. On the experiment of tumor LA,795, of mice, when the source power was 293.3 GBq and 96.2 GBq,no different biological effect can be seen between the two series of figures The relative biological effect was 0.55—0.60 (tumor growth delay) when compared with those of X-ray irradiation.