Corresponding to 4,633 diploid lymphocytes 64 tetraploid or near-tetraploid (including one octoploid) metaphases were found in two sibs with Bloom's syndrome. Eight of the polyploid cells had resulted from cell fusion with half the chromosomes representing PCC. One similar fibroblast was also observed. In 13,584 untreated cells from persons without a chromosome-breakage syndrome no such cells were encountered (P = 0.0000176). This seems to be the first description of spontaneous fusion of nonmalignant cells. The extended segments in the G2 type PCC which correspond to the Q-bright, late-replicating chromosome parts are less tightly spiralized than other chromosome regions which renders the chromosomes similar to those which have incorporated BrdU late in the S period. Obviously the chromosome segments which replicate last also are the last to spiralize.