Karyotypes of nine human colorectal cell lines deposited with the ATCC were studied by trypsin-Giemsa banding. CCL 229,230,231 and 235 (Modal chromosome number, Sm, was 49, 68, 47, and 40, respectively) belong in the stable type that is characterized by karyotypes consisting mostly of normal chromosomes and stable markers. CCL 228, 234, and 238 (Sm=55,79, and 70 respectively) belong in the unstable type that has karyotypes consisting of numerous markers in addition to normal chromosomes and stable markers. The remaining intermediate type (CCL 233 and 237, Sm = 60 and 64, respectively) has karyotypic characteristics between the above two types usually with two or less unstable markers per cell. The stable markers (together with normal chromosomes) are constitutive components of a cell genome and are common to most cells within the same cultured population. Unstable markers, which generally constitute only a small portion of the total chromosome complement are the likely cause of karyotypic variations between cells and often are produced by balanced inter- or intrachromosome changes, or both. Consequently, total chromosome length per cell genome is remarkably consistent within a cell population, and karyotypes between cells, such as from four stable lines, are profoundly stable and mostly identical. Chromosome deletions and interhomologue exchanges (including isochromosomes) had the highest incidences among both stable and unstable markers. The complex markers occurred relatively infrequently. There were neither common markers nor unique chromosome breakages common to all of these established cell lines. However, chromosomes No. 7 and 1 had the highest incidence (15 and 12, respectively) of structural modifications resulting in the formation of stable markers (82 total exchanges in nine cell lines), and chromosomes No. 7 and 2 were involved at high incidence (21 and 15, respectively) in the formation of both stable and unstable markers (181 total exchanges). Moreover, No. 7 is overrepresented in eight of nine lines. The significance of chromosome changes involving No. 7 in this as well as other tumor pathotypes is briefly discussed.