Modal karyotypes of mouse mutant cell lines, LM(tk-), A9, and Rag, which are frequently used as a donor line for somatic cell hybridization, are described and compared with karyotypes previously reported by other laboratories. Results are summarized as follows. Many cells (18%-38%) in each line had the same chromosome composition as the modal karyotype. Chromosome differences from the modal karyotype in some cells mainly involved one or a few chromosomes only. These differences are mostly caused by structural changes from well documented mechanisms or simple numerical changes including artifacts arising from the rupture of cells. Karyotypes of coexisting subpopulations within the same cell line can be clearly identified. Assessment of karyologic properties of a heteroploid cell population may vary considerably depending on whether it is estimated on a subpopulation or the total population as a unit. Modal karyotypes of these cell lines contained most marker chromosomes of, and a similar chromosome composition to those previously reported by other laboratories. These cell lines exhibit karyotypic constancy; thus, we believe that chromosome changes associated with clonal growth of somatic cell hybrids can be identified without ambiguity.