Adequate methods for monitoring any type of gametic mutation directly in man are virtually nonexistent. A method is presented by which one can monitor Y chromosomal nondisjunction directly in the male gamete by quantifying the number of spermatozoa with two fluorescent bodies (YFF) in 1000 sperm counted. Dried semen slides are stained with quinacrine dihydrochloride and examined under a fluorescent microscope with dark field illumination. This method eliminates the biopsy required for other meiotic studies and further eliminates bias in gametogenic selection by evaluating ejaculated mature spermatozoa. Since chromosomal numerical errors are found in 0.4% of term births and 35% of miscarriages, it is evident that chromosomal aneuploidy constitutes the major mutagenic load in man. In view of the increases observed in the incidence of YFF sperm in patients receiving antineoplastic therapy and in the DBCP-exposed workers, it may be prudent for men who have a history of exposure to mutagens and who are contemplating reproduction to be evaluated by this method prior to attempting conception. Further, this procedure could also be applied to the clinical phase of new drug testing to evaluate the effects of that agent with respect to aneuploidy since the increases in Y chromosomal nondisjunction may well act as a barometer for increases in overall autosomal nondisjunction.