The sex-chromosome constitution and the incidental rate of the 2A-XX/2A-XY chimerism were studied with colchicine treated bone marrow specimens, collected from three different bones in 3 freemartins and one male calf co-twined with a freemartin. Bone marrow cells in all cases manifested 2A-XX/2A-XY chimerism with varying degrees. But the chimera ratios tended to parallel each specimen in an individual and also with a set of co-twins. In addition, there is a similar tendency in the chimera ratios between bone marrow cells and cultured leukocytes
