A method for synchronization of bone marrow cells with fluorodeoxyuridine (FdU) is presented and compared with methotrexate (MTX) synchronization. FdU has the advantage of not requiring cell washing for release of the DNA synthesis block and was found to be more beneficial for bone marrow cultures because it generally produced a higher mitotic yield and was less damaging to chromosomes. Late-replication banding, produced after releasing the FdU block with bromodeoxyuridine (BrdU), indicated that cells in midsynthesis at the time of the block release were those that showed the most increase in chromosome length and the most improvement in the quality of the metaphase spread. Therefore, because bone marrow cells have a longer cell cycle time than stimulated lymphocytes, a minimum of 7-8 hr culture after release of the block is recommended to give optimal results. It was also found that, to increase the yield of mitoses, at least 6-8 hr of growth was necessary before the addition of either of these synchronizing agents.