A newly developed technique allows cytological (light microscope level) chromosome preparations to be examined at the electron microscopic level. Ultrathin (50 nm) sections of highly condensed Hela cell metaphase chromosomes show the characteristic mitotic chromosome morphology. In addition a fibrous network (presumably chromosome fibers) can be seen within them. Fibers appear to be gathered at foci along each chromatid. Treatment of chromosomes with trypsin in a trypsin/G-banding procedure reduces the amount of staining material at the electron microscopic level and results in more prominent foci. Thicker (100 nm) sections of less condensed chromosomes prepared from human lymphocytes display a banding pattern similar to G-banding, even without pretreatment with proteases.