We separated 40 bone marrow samples by centrifugation on two density gradients, ficoll-metrizoate sodium (lymphoprep) and percoll. There is no difference between the two methods when expressing the colony forming cells (CFU-C) per 2.10(5) bone marrow cells plated. But the absolute number of recovered CFU-C is significantly greater with percoll than with ficoll metrizoate sodium.