The distribution of chromosome breaks induced by streptonigrin (SN) and X-ray in chromosomes Nos. 1, 2, and 3 of cultured human leukocytes was investigated. In all chromosomes tested, the breaks caused by X-ray were randomly distributed both among and within the chromosomes in proportion to their relative lengths. SN, however, produced significant non-random distributions of breaks in chromosomes No. 1 and No. 2. The pericentric regions of chromosomes No. 1 and No. 2, as well as the secondary constriction area in the long arm of No. 1, seemed highly susceptible to breakage. Although SN produced significantly fewer breaks than expected in chromosome No. 3, these breaks were randomly distributed along its length. The telomere regions of all chromosomes and the short arm of chromosome No. 2 appeared to be relatively resistant to the action of SN.