Heterochromatic regions in chromosomes of man, mainly at the centromeric areas, can be demonstrated with consistency using a special staining procedure. This procedure includes treatments of cytological preparations with HCl, RNase, NaOH (denaturing DNA), and renaturing DNA in a saline citrate solution at 65°C. The heterochromatic regions can be observed after staining with Giemsa. These regions are believed to be composed of constitutive heterochromatin. Facultative heterochromatin regions, e.g., the repressed X in female cells, do not respond to this series of treatments.