The present experiment was performed to investigate the types and frequencies of X-ray induced chromosome aberrations in human leukocyted in vitro. Blood samples were obtained from 10 nurses working at Seoul National University Hospital. Ten ml. of heparinized blood were irradiated in 50 mm. Pyrex Petri dish, using General Electric Maxitron Ⅲ-A, Which was operated at 250 KV and 10mA. Five samples were given doses of 25R, another five 50R. The culture method was that of Moorhead et al, with slight modification. Leukocytes were isolated, and placed in culture bottles containing culture medium consisting of 6ml. of TC 199, 1 ml. of homologous human serum, 1ml. of plasma containing leukocytes of 8-12×106/ml and 0.03ml. of phytohemagglutinin-P form(Difco) with 800 units and 0.8mg. respectively of penicilline G and streptomysin. The culture were incubated at 37℃ for 72 hours. Colchicine was added to the culture 3 hours before fixation to a final of 10-7M. The cell were fixed, mounted, and stained with Giemsa solution. The results were compared to two cases of non-irradiated contol group. Results 1. The frequency of modal number in control group was 96.6%, whereas 96.57 for 25 R irradiated group and 95.55 for 50R group. The aneuploid cells were found 3.40 in control group, whereas 3.42 for 25 R irradiated group and 4.45 for 50 R group. 2. The distribution of chromosomes in irradiated group was similar to that of control group with slight increased frequency of aneuploid cells. 3. Chromatied aberrations were found both in control group and X-ray irradiated group, but no increase of frequency was observed after X-irradiation. 4. In X-irradiated groups chromosome aberrations, such as dicentrics, rings and deletions were found with an increase according to the X-ray doses given.