AbstractMicronuclei were assessed among the bone marrow PCEs of mice 28 hr after exposures to 1 to 8 Gy of X‐rays and at 6‐hr intervals from 12‐60 hr after exposures to 2 or 6 Gy. At 28 hr, the frequency of micronuclei declined as the exposure level increased from 1 to 8 Gy. The peak proportion of micronucleated PCEs appeared much later following 6 Gy than after 2 Gy exposures, implicating cell cycle delay as the cause of the negative dose‐response relationship.