Data obtained after various types of partial body irradiation support the concept of a small rapidly exchangeable pool of CFUs which seems to be exhausted rapidly after irradiation. The depletion of this pool is the most plausible explanation for the decrease in stem cell migration observed 3 hr after exposure in C3H mice. After partial body irradiation the size of the rapidly mobilizable pool is reduced in proportion to the areas of bone marrow irradiated. When only one marrow area is shielded, the recovery of this pool does not occur during the first 24 hr after exposure.