Two monoclonal antibodies, N/61 (IgG2b) and N/92 (IgG2a), reacting preferentially with blood group N antigen, were obtained by immunization of BALB/c mice with human red cell membrane glycophorins. The antibodies showed distinctly different specificities. N/61 reacted with an epitope with NH2-terminal Leu residue, its free amino group, and sialic acid residues as essential components. The antibody N/92 reacted equally well with untreated and desialylated N glycoprotein, but did not react with de-O-glycosylated antigen. It seemed to recognize an epitope including both blood group N-characteristic amino acid residues, namely the 1st Leu and 5th Glu. The antibodies also differed in pH-dependence. N/61 showed maximal activity at pH 6-7 with tendency to decrease at higher pH values, whereas N/92 was not or weakly active up to pH 7, and showed a strong increase of activity at pH range 7-8.