Chromosome examinations were made both in cultured peripheral lymphocytes and in freshly aspirated bone marrow cells of 6 persons accidentally exposed to gamma rays from an 192Ir. Samplings of peripheral blood for the chromosome analysis were repeated mostly at weekly intervals during the hospitalization beginning on 10-11 days after the initial exposure, and at more prolonged intervals after that period. The average yield of dicentrics plus rings in peripheral lymphocytes ranged from 0.0015 to 0.1367 per cell. The chromosome abnormalities, mostly of the stable-type rearrangements of chromosomes, were observed in bone marrow from two of the 6 cases. Based on the aberration yield in peripheral lymphocytes, the average total body absorbed dose was estimated to be equivalent to 9.8 to 124 rad of 60Co gamma rays and 15.6 to 152 rad of high energy X-rays. When the dose estimate was made with the Qdr value, the average total body absorbed doses were higher than those obtained by direct extrapolation of the aberration yield. This seemed to be resulted from non-uniformity of the exposure.