Cytogenetic analyses of 11,950 lymphocyte metaphases from 122 cultures from 10 control men and 17,759 lymphocyte metaphases from 183 cultures from 21 control women have shown: (1) variations in breakage frequencies in consecutive cultures from the same person, (2) variations in average breakages among cultures from different people, (3) greater variability in breakages in cultures from women than in cultures from men, and (4) differences in average breakage frequencies in cultures obtained at different times of the year.