The Ina and Inb antigens have recently been added to the list of diverse erythrocyte blood group antigens whose expression is down-regulated by the In(Lu) gene. Evidence has been provided that these antigens constitute polymorphisms of an erythrocyte glycoprotein previously defined by its reactivity with murine monoclonal antibodies, such as A3D8, and designated In(Lu)-related p80. This study has now confirmed the assignment of the Inb antigen to p80 and has further studied the relationship of the Inb epitope to other p80 epitopes recognized by both monoclonal antibodies as well as polyclonal antiserum produced against purified erythrocyte p80.