A gene controlling the expression of a polymorphic 92,000-dalton glycoprotein of mouse macrophages and granulocytes has been identified. This glycoprotein was previously shown to be the major iodinated, trypsin-sensitive component of the murine phagocyte cell surface. The gene has been provisionally designated Pgp-1 for phagocyte glycoprotein 1. Expression of the glycoprotein was measured by monoclonal antibody binding to a polymorphic antigenic determinant. Antibody binding to cells of positive strains of mice was proportional to cell number, whereas binding to cells of negative strains was insignificant. The concentration of the antigen in cells of heterozygous mice was approximately 50% of that in homozygous mice. Thirteen of 33 recombinant inbred strains of mice were positive, with binding values 100-fold over background, suggesting that a single gene controlled expression of the antigen. Segregation of the antigen correlated with markers on chromosome 2. The segregation of Pgp-1, with nonagouti coat color (a) and hemolytic complement (Hc) activity among progeny of (C3H/HeJ x DBA/2J)F1 x DBA/2J mice confirmed the single gene control and the chromosomal assignment. Another gene on chromosome 2, Ly-m11, was also typed by using (BALB/cJ x C57BL/6J)F1 x C57BL/6J mice. The data from both of these crosses indicated the following gene order: Hc--Pgp-1--Ly-m11--a.