A genetic assay is described in which a mouse/human hybrid cell line R3-5 containing a single human chromosome (a monochromosomal hybrid) is used to detect chemically induced aneuploidy. The hybrid cells are deficient in hypoxanthine guanine phosphoribosyltransferase (HGPRT) and contain human chromosome 2, marked with Ecogpt, an E. coli gene for xanthine guanine phosphoribosyltransferase. These cells with a genotype of HGPRT(sup -)/Ecogpt (sup +) can grow in the medium containing mycophenolic acid and xanthine (MX medium) but not in the medium containing 6-thioguanine (6-TG). The loss of the human chromosome from R3-5 cells as a result of chemical treatment produces cells with a genotype of HGPT(sup -)/Ecogpt(sup -) which are capable of growth in the medium containing 6-TG. Thus, the cloning efficiency of cells treated with a test chemical in 6-TG provides a method to determine the frequency of cells that have lost the human chromosome. Colcemid and nocodazole, known to induce aneuploidy at concentrations ranging from 0.002 to 0.032 micrograms/ml, showed a concentration-related positive response in this assay.(Copyright (c) 1988 Elsevier Science Publishers B.V.)