Common chromosome break points were found in a banding study of 25 chromosomally abnormal fibroblast clones derived from normal individuals and patients exposed to environmental carcinogens. Of 81 break points, 38 % appeared to be in terminal bands. This specificity did not appear to be the consequence of the biased recognition of exchanges. In addition, specificities for break points at exact interstitial loci were detected. The presence of clones in cultures grown in a wide variety of conditions, the finding of two identical clones in two independent lines from the same biopsy, and the absence of any obvious continuing instability in lines support the view that chromosomally abnormal fibroblasts occur in vivo.