An account is given in this paper of a method of identifying the fluorescence body (f-body) as a marker of the Y-chromosome. Also covered by this method is poor decondensation of spermatozoal nuclei when exposed to action of 1.25% of papain, 0.155% of DTE, and 0.025% of DMSO. Quinacrine mustard was used as fluorescent stain, its final concentration being 0.0025%. For staining, spermatozoa were suspended for 1-5 h. Average f-body frequency accounted for 41.0 +/- 5.1% in 35 ejaculates from 22 bulls. The overall variation coefficient amounted to 12.4% and thus was higher than each of the single values individually recorded from six bulls which were involved with three or four ejaculates (3.6-7.0%). F-bodies could not be detected by the method generally used on human spermatozoa. The applicability of the f-body test to quantification of Y spermatozoa in experimental separation of androspermatozoa and gynospermatozoa is discussed and is demonstrated by an example.