Following upon earlier work the response of MCF-7 breast cancer cells to mitomycin C, a known genotoxicant, was examined. This previous research has yielded a baseline of chromosome aberrations in human peripheral blood lymphocytes from which comparisons can now be made. Results of genotoxicity testing conducted with the same protocol yielded a significantly increased level of chromosome aberrations in MCF-7 cells than in normal human peripheral blood lymphocytes. Dose response curves are presented. Although these results need to be confirmed and extended, the hypothesis is proposed that the present findings are simply another manifestation of the genetic instability inherent in these cancer cells. This seems to be the only known published study of breast cancer cells addressing this issue using the techniques of cytogenetics.