Aspects of the granulocyte-macrophage progenitor cell (CFU-GM) assay not previously used in the content of bone marrow transplantation were discussed in terms of the additional information which could be derived. This focused on the characteristics and responsiveness of the progenitor cells to regulatory activities and other agents. In addition, Ia-like antigenic determinants were demonstrated on human "pre-CFU-GM", an assay which can be used to assess what appears to be a more immature progenitor cell.