ATTEMPTS have been made to distinguish between spermatozoa of mammals containing either an X or a Y chromosome by looking for differences in size and surface properties of the two classes of cells (see refs. 1 and 2 for reviews), but the results of such approaches are equivocal. Another approach is to try to identify the X or Y chromosome within the head of the spermatozoon. Like Zech3 and others4,5, we have found that the distal end of the long arm of the Y chromosome in mitotic and meiotic meta-phase nuclei of human cells displays a fluorescence more brilliant than that shown by the other chromosomes when stained with either quinacrine or quinacrine mustard. This property is carried by the Y chromosome into the chromatin of the mature spermatozoon.