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Out of 112 offspring obtained from crosses between X-irradiated males and non-irradiated females, nineteen had chromosome abnormalities. There were 11 reciprocal translocations between autosomes, two inversions, two complex translocations, one deletion of Xq, one isochromosome of Xq, one marker chromosome and one numerical abnormality (23, XXX). The marker chromosome was chromosome 6 bearing an extra segment on the short arm. Among animals with these rearrangements, two males with complex translocations, a female with a deletion of Xq and a female with an isochromosome of Xq, were sterile. On the other hand, the 23, XXX female was fertile, but all the offspring were karyotypically normal (22, XX or XY). Among offspring of other fertile animals, male and female heterozygotes with the same chromosome rearrangements as the parents were all fertile. Strains with these chromosome rearrangements are presently maintained by the authors. These animals serve as useful materials for various cytogenetic studies.