Human peripheral leukocyte cultures 66 hours old have been irradiated with X rays and fixed 6 hours later. The cells were scored for chromosomal aberrations. The aberrations seen were mainly of the chromatid type, and included all of the expected types. The average coefficient of breakage production for four experiments was 0.0031, or virtually the same as that found earlier (Bender and Gooch, 1962a) for chromosome-type aberrations induced in the human leukocyte system.