The epitopes of seven mouse monoclonal antibodies which are related to the Gerbich blood group system were investigated. BRIC4, BRIC10, GERO and MR4-130 have been published earlier. The three others (APO1, APO2, APO3) were prepared by immunization with normal human erythrocytes and detected by screening with red blood cells that lack glycophorins C and D. Using immunoblotting and hemagglutination inhibition assays, the epitopes for all antibodies were found to be located on glycophorin C. Hemagglutination inhibition experiments with peptides and chemically modified glycophorins revealed that MR4-130, GERO, BRIC10 and APO2 are all directed against identical or rather similar epitopes comprising the N-terminal three or four residues of glycophorin C. Modification of the N-terminal methionine residue or release of sialic acid attached to oligosaccharide(s) at the third and/or fourth position(s) destroyed all these antigens. The epitope of APO3 was found to comprise glutamic acid17 and/or aspartic acid19 as well as the oligosaccharide attached to serine15. The antigens of BRIC4 and APO1 were found to be located within the residues 2-21 and to comprise sialic acid attached to O-glycosidically linked oligosaccharide(s). However, these epitopes could not be elucidated further. Radio-iodinated MR4-130 bound to 39,000 receptor sites per normal red blood cell. Binding of the labelled antibody was completely inhibited by unlabelled MR4-130, BRIC10, APO2 and GERO. APO1 caused partial inhibition suggesting that it is directed against an adjacent site. BRIC4, APO3 and anti-Ge3 did not inhibit the binding of labelled MR4-130 to any significant extent.