Scanning electron microscopy (SEM) has been utilized for the past several years for the study of whole-mount preparations of human chromosomes. Cell cycle-dependent changes in chromatin can be demonstrated easily. Chromomeres, which are mass accumulations at complementary points along each chromatid, provide a basis for the "banding" patterns produced by various stains used in light microscopy. In a highly condensed metaphase chromosome, only a rare free fiber end is seen, suggesting that a single chromatin fiber folds into a chromatid. The presence of interchromosomal fibers suggests that the DNA molecule (i.e., the chromatin fiber) might be folded into more than one chromosome. The specificity of the 9q+/22q-Ph1) translocation in chronic myelogenous leukemia and the evidence for nonrandom translocation abnormalities in adult acute leukemia suggest that either the linear integrity of the chromatin fiber comprising several chromosomes is real or that the nuclear membrane attachment site of individual chromosomes results in specific, adjacent chromosomes which are available for induction of nonrandom translocations.