Four groups of male LACA mice were injected intraperitoneally with radioactivity of HTO (Bq×10,5,/g•b•w) :8.4、16.8、29.3、45.2, respectively; the accumatve doses absorbed by testis in 10 days aftr injecton were 0.19, 0.38, 0.66 and 1.01 Gy accordingly. Meanwhile, three groups of male mice were exposed to ,60,co gamma rays for 10 days in decreasing dosage following the exponentially decreasing trend of tritium concentrations in testis; the accumulative doses were 0.743, 1.592 and 2.074 Gy, respectively. The preliminary results showed that the yield of induced dominant lethal mutations ranged from 0.086 to 0.149 for tritium treated animals, and that for gamma irradiated mice was 0.0436 to 0.303; in both cases a linear dose-effect relationship was observed. Based on the above experimental results, the estimated RBE value for tritium is 2.1.