The differences between colony promoting activity (CPA) and colony stimulating activity (CSA) in the culture media of murine long-term bone marrow cultures (LTBMC) were demonstrated and the role of adherent cells and nonadherent cells in the production of CPA was studied in this culture system. Supernatant harvested from intact continuous marrow cultures showed high CPA but contained no CSA. Assayable CSA was detected in concentrated supernatant. However, there was no significant relationship between levels of CPA and CSA in the supernatant. When adherent cells and nonadherent cells from LTBMC were separately cultured, CPA was detected in the conditioned medium of adherent cells but not in that of nonadherent cells. The CPA level in LTBMC was related inversely to the number of nonadherent cells and addition of nonadherent cells to adherent cell cultures reduced the level of CPA. Conditioned medium of nonadherent cells showed no inhibitory activity of CPA. These results indicate that CPA is produced by bone marrow adherent cells and that it may be consumed by myeloid progenitor cells in nonadherent cells.