The proportion of cells with clonally derived chromosome rearrangements has been recorded at different passage levels in 16 lines of human fibroblast cultures derived from adults. All abnormal clone populations recognized at the lowest passage levels showed a stable or declining frequency relative to the normal cells, although 2 out of the 21 clones detected did show a transient increase. Only 2 clones were found at higher passage levels that were not previously identified at low passage. The observed selection against cells with rearrangements may reflect a general property of chromosome organization which ensures that cytogenetic abnormalities do not normally accumulate in rapidly dividing tissues.