In tissue culture high proliferation potential colony forming cells (HPP-CFC, Bradly & Hodgson, 1979) formed large colonies in which macrophages predominated. Granulocyte/macrophage colony forming cells (GM-CFC, Bradley & Metcalf, 1966) formed smaller colonies of granulocytes and macrophages. We compared HPP-CFC from normal adult mouse bone marrow with GM-CFC from the same source. We found that HPP-CFC differed from GM-CFC in the following ways: (1) HPP-CFC formed larger colonies containing more than 5 X 10(4) cells. (2) They required spleen conditioned medium (Metcalf & Johnson, 1978) as well as pregnant mouse uterine extract (PMUE, Bradley et al, 1971) as stimulants. (3) Only 9% of HPP-CFC were killed by the phase specific drug hydroxyurea. 36-55% of GM-CFC were killed by the same treatment. (4) The modal density of HPP-CFC was 1.070 g/cm3. That of GM-CFC was 1.080 g/cm3.