A mouse hybridoma has been isolated which secretes a hemagglutinating monoclonal antibody (MoAb) recognizing the N and 'N' blood group antigens. This conclusion is based upon the following observations. First, all red cells expressing either N or one of the alleles of Ss (ie, 'N' were strongly agglutinated by the MoAb diluted fourfold. The only cells not reactive were of the M+N-S-s-(U-) and M+N-S-s-(U+) phenotype and cells (J.R. and A.G.) expressing Lepore-type hybrids of the MN and Ss sialoglycoproteins, which do not express N or 'N'. Secondly, red cells of the N+S-s-(U-) phenotype were rendered unreactive to MoAb following treatment of the cells with trypsin, which is known to destroy N antigen activity. Conversely, cells expressing S and/or s maintained their reactivity with MoAb following trypsin treatment, which does not cleave 'N' from the Ss sialoglycoprotein. When spent culture medium containing MoAb was diluted and tested against a panel of red cells, the antibody titer fell into two distinct categories depending upon the MNSs phenotype of the target. Red cells expressing either homozygous or heterozygous N-sialoglycoprotein (N-SGP) were agglutinated by 128-fold diluted MoAb. In contrast, a 16-fold dilution of MoAb was the endpoint for agglutination of cells lacking N-SGP, but expressing S and/or s.