With the use of adult rabbits, the author performed the bonemarrow culture in coverslips and obtained from the rabbits irradiated repeatedly and systemically with 100r of X-ray 24-41 times, and 300r of X-ray ten times, and 1,000r of X-ray once and observed the bone marrow from respective groups during the periods ranging 1-30 clays after the irradiation. The results are described in the following. 1) In the group receiving repeatedly and systemically the irradiation of 1000r of X-ray the relative growth rate, cell density, the wandering velocity of pseudoeosinophils all decreased markedly. 2) In the group given the irradiation of 300r X-ray, no sign whatsoever could be recognized as regards the relative growth rate, cell density and the wandering velocity of pseudoeosinophils; and the bone marrow presented aplasia. 3) In the group receiving 1,000r of X-ray irradiation once only, the relative growth rate, cell density and the wandering velocity of pseudoeosinophils all docreased gradually lower with the lapse of time; and all the values presented their minimum on the tenth day but on the thirtieth day it was found that they were all rather increased. 4) Of the disturbances in the bone marrow due to the X-ray irradiation. in the groups receiving repeated irradiation the cell proliferation was disturbed in direct proportion to those of the cell function. In the group receiving a single irradiation of 1,000r, the cell proliferation was disturbed on the second day but it seemed to have a tendency to precede the disturbances of the cell functions. Therefore, it is assumed that in the case receiving a small dose of X-ray irradiation the cell proliferation of bone marrow is disturbed earlier, and the cell functions are diminished later.