A total of 130 stable, two-break reciprocal translocations were scored in G-banded karyotypes prepared from 375 metaphase spreads from a strain of human diploid fibroblasts irradiated with 400 or 600 rads and analyzed 1-20 mean population doublings later. The chromosomal location of each of the 260 breakpoints was mapped. The sites of 121 chromosomal breaks and deletions in the first postirradiation mitosis were also scored. Unlike the random distribution of these latter events, the translocation breakpoints showed not only a nonrandom distribution among chromosomes but also the existence of specific sites within chromosomes that were more frequently involved in translocations. The most notable finding was a marked excess of translocations involving the short arm of chromosome 1, in particular, band 1p22. The specific types of translocations were random, although the breakpoints were not. Eight of the 12 most frequently involved chromosomal sites were regions in which fragile sites have been mapped in human lymphocytes.