Male mice were hypodermically injected with human liver RNA, then exposed to 1.5 Gy X-radiation 7 hours or 24 hours later, the bone marrow cells were tested using the method of chromosome aberration. The results showed that human liver RNA could significantly reduce the frequencies of chromosome aberrations in bone marrow cells of mice 7 hours after exposed to 1.5 Gy X-rdiation. The best injection time of human liver RNA was 24 hours prior to irradiation and the best dose was 25 mg/kg. 25 mg/kg human liver RNA could also reduce the frequencies of chromosome aberrations in mice 24 hours after exposed to 1.5 Gy X-radiation. This function might be related to interferons induced by injection of human liver RNA.