An ,241,Am-Be neutron source of 120 millicrocurie was stolen by a boy on April the 3rd, 1982. He brought it to his home. The neutron source was broken and the radioactive substance contaminated the whole family. He and his other family members did not leave the contaminated environment for 70 days. There were altogether 14 persons exposed to the radioactive substances.,Chromosome aberrations of peripheral blood lymphocytes of six exposed cases were analysed on an early day of June 1982. The exposure doses were estimated by the frequency of chromosome aberration. Biological dose absorbed by the boy who stole the source was about 96~128 mGy. (physical dose abous 120 mGy), the others were about 10~30 mGy (physical dose about 10~30 mGy). The results indicated that biological doses were quite approximate to the physical doses.,One and half years after the accident, the analysis of stable chromosome aberrations for 14 exposed persons showed that the frequencies of chromosome aberrations of the irradiated individuals still were higher than those of the controls. The differences between them are very significant.