We have previously shown that the In(Lu) gene down-regulates expression of an erythrocyte protein antigen identified by murine monoclonal antibody (MoAb) A3D8. In the present study we have examined In(Lu) Lu(a-b-) erythrocytes for expression of additional epitopes on the erythrocyte 80 kilodalton protein (p80) bearing the A3D8 antigen. Using a total of seven additional MoAbs that recognize three epitopes on erythrocyte p80, we have shown that In(Lu) Lu(a-b-) erythrocytes exhibit down-regulation of expression of all three epitopes. In(Lu) erythrocytes also showed a reduction in their reactivity to rabbit antibodies produced against purified p80 from either erythrocytes or lymphocytes. Furthermore the reactivity of the MoAbs was not altered by treatment of the cells with neuraminidase but was substantially reduced by treatment of cells with trypsin or chymotrypsin. The polyclonal anti-p80 sera were shown to react with a fragment of 50,000 daltons, still associated with erythrocyte ghosts, following treatment of the cells with trypsin or chymotrypsin. Treatment of erythrocytes with the thiol-reactive reagent AET decreased their reactivity with the MoAbs but had a variable effect on their reactivity with polyclonal antibodies. Erythrocyte p80 is a glycoprotein with N-linked oligosaccharides, as demonstrated by its binding to concanavalin A (Con A) and Len culinaris lectins. Following Endoglycosidase F treatment, erythrocyte p80 underwent a reduction in apparent mol wt of 11,000. The presence of a reduced amount of an intact p80 glycoprotein, seen by a decrease in reactivity with MoAbs directed at three distinct epitopes and with two different polyclonal antibodies, suggests that the In(Lu) gene interferes with expression by erythrocytes of the entire p80 glycoprotein.