Using Chinese hamsters heterozygous for T(2;10)3Idr and T(1;3)8Idr reciprocal translocations, the authors studied mitomycin C(MMC)-induced crossing-over on the interstitial segments. Marker chromosomes with unequal-length chromatids resulting from crossing-over were clearly detectable, and the frequencies of such marker chromosomes were constant among individual males which were heterozygous for the same reciprocal translocation. The frequency of MMC-induced crossing-over on the interstitial segments increased roughly with increase in dose. These findings, therefore, indicated that marker chromosomes with unequal- length chromatids in translocation heterozygotes may be a useful indicator for detection of the cytogenetic effects of environmental mutagens on germ cells.