Tritiated thymidine was added to peripheral blood cultures containing phytohemagglutinin so that DNA synthesis in interphase nuclei of white blood cells in the human male could be studied. After 57 hours in culture, a large heterochromatic body with a central position is seen in unlabeled Feulgen-stained nuclei. In labeled nuclei in which DNA synthesis was taking place in both the eu- and heterochromatin at the time the thymidine became available, the heterochromatin shows a higher number of silver grains per unit area, accompanied by a stronger Feulgen reaction, an indication of its higher DNA content. The time of DNA synthesis in the heterochromatin blocks is different from that in the surrounding euchromatin. The large heterochromatic block is composed of chromosome segments gathered together around the nucleolus but it is not part of this organelle. In preparations stained with azure A and acid fuchsin for demonstrating both the nucleolus and the chromosomes, six distinctly heteropyenotic chromosome segments can be seen associated with the nucleolus. Cells of all size categories were found to incorporate tritiated thymidine. The distinct appearance of autosomal heterochromatin in white blood cells may be the result of the new physiological conditions to which the cells are subjected in the medium containing phytohemagglutinin.