Aneuploidy is one of the genetic changes observed in many tumors. However, it is not known whether monosomy in a single chromosome mediated by micronucleus formation is a viable mechanism leading to genetic disease. To address this question, we treated L5178Y mouse lymphoma cells with four aneugens. We found that, although they induced micronuclei containing predominantly whole chromosomes, they did not induce mutations in these cells at Tk1 under conditions where they induced micronuclei. This suggested that the induction of micronuclei containing whole chromosomes was not an early event leading to phenotypically expressed mutations in these cells. If chromosomes were not randomly excluded from the nucleus to form a micronucleus, it is possible that the numbers of chromosome 11 in the micronuclei might be low. To examine this possibility, we painted micronuclei with four mouse chromosomes, chr. 1, 11, 19 and X, and counted the numbers of micronuclei that harbor a specific chromosome. Our analysis indicates that the numbers of chromosome 11s in the micronuclei are sufficient to induce enough mutations at Tk1 to be observed. We have also identified a number of compounds that induce micronuclei containing predominantly chromosomal fragments. We isolated seventy-five etoposide-induced mutants and, thus far, analyzed twenty-five for loss of heterozygosity. Twenty-three of them show extensive loss of heterozygosity and thus point mutations are not responsible for their mutant phenotype. The remaining two show no losses at the locus closest to Tk1 that we examined. Based on the LOH data, these mutants could contain deletions or mitotic recombinations. If these mutants contain deletions, then we would have evidence that micronuclei containing chromosomal fragments may be precursors to chromosomal mutations. However, we know that L5178Y mouse lymphoma cells can also detect mitotic recombination and gene conversion events. Such lesions would also exhibit loss of heterozygosity but would not be consistent with micronuclei containing chromosomal fragments.