THE size distributions of spermatozoan nuclei are of interest from two points of view. On one hand there is the question of the identification and separation of X and Y chromosome bearing spermatozoa, and on the other hand the relation of nuclear size to total content of chromatophilic substances measured by the plug or core method of microspectroscopy. We do not wish to become involved in the former question; but are interested in the latter in connexion with work on the DNA content of bovine spermatozoa being examined in this laboratory.