MG-1: a specificity identifying members of the macrophage and granulocyte lineages of mice.

An antigen (MG-1) which behaves as a lineage marker for immature granulocytes and cells of the monocytic series in mice is described. It is identified by rabbit anti-mouse brain antiserum, which has been exhaustively absorbed with thymocytes, red blood cells, and a Thy-1 negative variant of a T-cell lymphoma. MG-1 is present on immature granulocytes, declines in its surface expression as the cells differentiate and is absent from the most mature cells (segmented) of the series. Early monocytes are strongly positive for MG-1 and, with maturity, the amount of cell surface antigen increases. Adherent, phagocytic macrophages are brilliantly positive when stained with anti-MG-1 antiserum in an immunofluorescence assay. The antigen is present on most of the adherent cells of the lung, spleen and peritoneum. Many multipotential stem cells also express low levels of this antigen as do some bone marrow B cells.