BALB/c mice were immunized by intraperitoneal injections with human blood group substances; 4 mice with M and 4 with N. Immune spleen cells were fused with murine myeloma cells X63-Ag-8.653. Two clones secreting monoclonal anti-M and seven secreting monoclonal anti-N were identified. All antibodies were of IgG1 subclass and had kappa light chains. Three clones have been maintained that produced antibodies useful for typing purposes: anti-M, A09 originating from a mouse injected with M, anti-N, AH7 originating from a mouse injected with N, and anti-N BO10 originated from a mouse injected with M substance. In typing 370 erythrocyte samples, monoclonal reagents gave identical results with commercial anti-M or anti-N typing sera of rabbit origin. Significantly, anti-N reagent AH7 obtained by immunization with N substance showed serological differences from anti-N reagent BO10 obtained by immunization with M substance in that AH7 had apparently higher avidity to N specificity on glycophorin A of N erythrocytes than BO10, whereas BO10 showed higher avidity for 'N' specificity on glycophorin B of M and N erythrocytes than AH7. These two reagents showed also somewhat different patterns of reaction with enzymatically digested erythrocytes. This apparent serologic difference between N and 'N' specificities is at variance with current immunochemical data.