A nondisjunction and chromosome breakage screening system devised by Craymer and modified in our laboratory, involves an X;Y translocation with the short arm of the Y (Ys), marked with the wild type allele of yellow, attached to the distal end of an X (break point 11D) carrying the recessive marker y; and the long arm of the Y chromosome (YL), marked with the dominant locus Bar of Stone (BS), attached to the proximal end of the X. A female tester strain carrying normal chromosomes homozygous for the yellow allele is employed in the mating scheme. Following normal disjunction in the male, all zygotes, which in this case receive aneuploid paternal sex-chromosomes and a normal euploid maternal complement, will die as a result of genetic imbalance. Thus all survivors from this corss can be classified as exceptions arising from: (1) nondisjunction in the female; (2) gross deletion of the paternal X;Y chromosome; (3) complete loss of the paternal X;Y chromosome; or (4) primary meiotic nondisjunction in the male. Results indicate the sensitivity of this scheme for the detection of events induced by x-rays and various chemicals. Positive results have been obtained with the known mutagens EMS and x-radiation.