The chromosome constitution of a number of cell strains derived from mouse tissues was studied in detail. Strains H1a, H1b, H1e, and H4a were heteroploid (with chromosome numbers in the subtetraploid range) when the first samples were taken. At that time they were 9 to 11 months of age in vitro. Strain H4c, initiated and sampled at the same time as strain H4a, was primarily diploid. Strain H2 was principally diploid 5 months after its primary cultivation. In its control line, H2c, tetraploid, and later subtetraploid, cells gradually gained dominance and became the stemline. In populations of strain H2, treated intermittently with 5-bromodeoxyuridine, polyploidization was accelerated in one line (H2a). In another line, H2b, a temporary reversion to near diploid (s = 38) was noted. However, cells in the subtetraploid region finally became the stemline also. Thus heteroploid transformation may undergo the following 2 patterns: diploidy-tetraploidy-hypotetraploidy and diploidy-subdiploidy-hypotetraploidy. Structural changes of chromosomes were constantly observed in all cell lines, especially those with heteroploid stemline. Most biarmed chromosomes had equal arms, suggestive of isochromosome formation. However, few, if any, metacentrics were incorporated into the stemlines of the strains analyzed. The thymidine analogue, 5-bromodeoxyuridine, induces chromosome breakages and accelerates polyploidization.