Peripheral granulocytes from rheumatoid arthritis and reactive arthritis patients were recently found to express higher levels of a newly defined Ag, termed M6, in comparison to granulocytes from healthy subjects. We present here the molecular characterization of M6 Ag and show that it is a novel human leukocyte activation-associated cell surface glycoprotein. Peripheral lymphocytes do not significantly express M6 Ag, however, it appears upon 3-day PHA-activated T blasts. On monocytes, which constitutively express M6 Ag, it is down-regulated on day 1 but re-induced on day 3 of granulocyte-macrophage CSF stimulation. SDS-PAGE analysis of M6 immunoprecipitates shows a single band of 54 kDa under nonreducing conditions that shifts to 65 kDa under reducing conditions. Endoglycosidase F treatment of M6 immunoprecipitate reveals that 50% of the M6 molecule is composed of N-linked carbohydrates. By modifying the COS cell cloning strategy, we have isolated cDNA clones encoding M6 Ag. M6 cDNA hybridizes with a single mRNA transcript of approximately 1.7 kb in Northern blotting. Comparison analysis of the M6 sequence indicates that M6 Ag is a member of the Ig superfamily and the species homologue of rat OX-47 Ag, mouse basigin (gp42), and chicken HT7 molecule. The highly conserved remarkable transmembrane domain suggests that the M6 Ag may be a component of a multichain complex in the plasma membrane.