[en] X-ray induced chromosomal damage in bone-marrow tissue of male mice was studied using micronucleus technique. Dose response relationship was evaluated. Male Swiss mice received whole body x-ray irradiation at different doses from 25-1000 rads. Animals were sacrificed at the end of 24 hours, bone-marrow smears were made and stained in May-Grunwald-Giemsa. The preparatians were scored for the following types of aberrations: micronuclei in young erythocytes-polychromatic cells and in the mature erythrocytes-normechromatic cells. A dose dependent increase in the frequency of micronuclei in polychromatic cells up to a dose of 100 rads was observed. In addition the effect of post-irradiation duration on the frequency of micronuclei in polychromatic and normochromatic cells were studied. Male Swiss mice were exposed to 200 rads x-rays and were then sacrificed at different time intervals after irradiation and bone-marrow preparations were made and scored. Maximum polychromatic cells with micronuclei were observed in 24 hours post-irradiated animals, thereafter a decrease in the frequency of polychromatic cells with micronuclei was observed in 40 hours post irradiated animals. (author