Quinacrine mustard interphase fluorescence may be exploited to study Y and X chromosomal ploidy of many human tissues. Acetic alcohol-fixed cryostat tissue sections are superior, but formalin-fixed sections may also be used after the formalin is removed by vigorous washing in water. The percentage of cells of presumptively euploid female tissues which exhibit Y-like bodies is low, with means of less than l0% of all tissues except brain. In contrast, the mean percentages for euploid male tissues range between 50 and 63% (except for heart) and 30 and 40% when fixed in acetic alcohol and formalin, respectively. The differences between male and female tissues with either method of fixation were statistically significant. Nuclei with two fluorescent bodies were rare. The Barr body could also be recognized in all female tissues, and its presence could be quantitated in nuclei of skeletal and smooth muscle. This method permits the prospective and retrospective assessment of sex chromosomal ploidy, and the correlation of localized tissue karyotype with tissue function in individuals mosaic for aneuploidy of the Y.