The development and utilization of a monochromosomal hybrid cell assay for detecting aneuploidy and chromosomal aberrations are described. The monochromosomal hybrid cell lines were produced by a two-step process involving transfer of a marker bacterial gene to a human chromosome and then by integration of that human chromosome into a mouse complement of chromosomes through microcell fusion. For chemical induced aneuploidy, the segregation of a single human chromosome among cause chromosomes is used as a cytogenetic marker. The genetic assay for aneuploidy is based on the ability of cells to grow in a medium that selects for the loss of the human chromosome. The assay for clastogenicity is based on survival of cells after treatment with chemicals in medium that selects for retention of the human chromosome but loss of its segment containing DT locus. The assays greatly simplify the detection of chromosomal aberrations induced by environmental factors at low-dose levels.