Two collections of XY-containing karyotypes were screened to obtain every karyotype in which the Y chromosome was distinguishable from the other small acrocentric chromosomes. The metaphase figures corresponding to these karyotypes were then examined, and approximately one-fourth of these were discarded because of marked distortion of the chromosome spread. The 37 remaining metaphase figures, 22 from the Columbia-Presbyterian collection and 15 from the Southbury collection were used to study the frequency of peripheral location of specific chromosomes. Five circular metaphase figures from each collection were selected for measurement of the distance of specific chromosomes from the center. In each collection the five metaphase figures were of similar area, but the Southbury metaphase figures were approximately 25 per cent larger in area than the Columbia-Presbyterian metaphase figures. All metaphase figures were obtained by flattening cells from human leucocytes cultures that had been treated with colchicine and hypotonic sodium citrate prior to fixation.