Mouse Ly-6 proteins are characterized by lineage-restricted patterns of expression on lymphoid cells. A mAb (1A8) was produced to Ly-6G, a newly described member of the Ly-6 locus. Based on selective reactivity to cloned Ly-6 gene products expressed in EL4J cells, 1A8 was determined to be specific for Ly-6G. Furthermore, mAb to other Ly-6 specificities did not bind to Ly-6G-transfected EL4J cells, indicating that Ly-6G is distinct from other serologically defined Ly-6 specificities. FACS analysis using 1A8 demonstrated that Ly-6G was expressed in bone marrow but not substantially on other lymphoid tissues, including activated T and B cells. In the bone marrow, Ly-6G expression was primarily restricted to the cells with more forward angle light scatter, which are mostly granulocytes. The RB6-8C5 mAb, previously described to detect a myeloid-restricted Ag (Gr-1) on more differentiated granulocytes, also reacted with Ly-6G- and Ly-6C-transfected EL4J cells. Both 1A8 and RB6-8C5 selectively precipitate a M(r) 21 to 25 kDa, glycosylphosphatidylinositol-anchored protein. Collectively, these data indicate that the Gr-1 Ag is a member of the Ly-6 family and further link expression of individual Ly-6 genes with distinct lineages in mouse bone marrow cells.