A simple method is described by which sex chromatin can be studied in the cells of the external root sheath of single hairs. From a human capital hair many thousands of scorable nuclei can be obtained. Since this material is suitable for orcein staining, preparations are ready for analysis within a few minutes. Qualitative and quantitative results are presented from control persons and multiple-X individuals. The method is most convenient for studies in various mammals, and samples of results are given for some rodent and carnivore species.