Pilated gonococci added to human erythrocytes apparently bind to polypeptide band 3 and inhibit chloride–bicarbonate exchange across the membrane in contrast with nonpilated organisms. Known covalent inhibitors of anion transport (4,4′-diisothiocyano-2,2′-disulphonic acid stilbene and α-cyano-4-hydroxycinnamic acid) inhibit chloride efflux from erythrocytes and also reduce hemagglutination titres, strongly suggesting that band 3 is the major host-cell receptor involved in all interaction between gonococci and erythrocytes. In studies of other cell species, however, band 3 prepared from human erythrocytes inhibited adherence of pilated gonococci to human foreskin and HeLa cells, but was without effect in human buccal, Vero, and mouse L cells.