In the chromosome complement of Bos taurus, the large X and small Y are both metacentric while the remaining 29 autosomal pairs are all acrocentric, a karyological characteristic which permits unequivocal differentiation of 2 A-XX cells from 2A-XY cells at mitotic metaphase. In this study, seven Holstein-Freisian calves less than one day old were utilized. One bull and one heifer, normal and single-born, served as controls. In the remaining three freemartins and two bull twins, blood chimerism was determined cytologically. In the testes of the two bull twins, 2 A-XX cells were also detected. In fact, twice as many 2 A-XX as 2A-XY cells were found in the histologically normal testis of one animal. Whether or not these 2 A-XX cells were germ cells capable of functioning as spermatogonia could not be ascertained. In the free-martin gonads, there was no evidence of propagation of 2 A-XY cells.