Monoclonal antibodies BRIC 18, BRIC 68, BRIC 107 and BRIC 203 recognize high-frequency epitopes absent from erythrocytes expressing the Ko phenotype. BRIC 107 has anti-k (K2)-like specificity. BRIC 203 has a unique specificity denoted anti-Kpbc. All four monoclonal antibodies identify an M(r) 95,600 erythrocyte membrane protein by immunoprecipitation from radio-iodinated erythrocytes. In quantitative binding studies using IgG it is estimated that there are from 2000 (BRIC 18) to 4000 (BRIC 68) copies of the Kell glycoprotein per erythrocyte. Using Fab fragments the estimates are in the range 4000 (BRIC 18) to 18,000 (BRIC 68) copies. In competitive binding assays the four epitopes defined by the BRIC monoclonal antibodies fall into two non-overlapping groups. The first group comprises BRIC 18, BRIC 68, BRIC 203 and an antibody (6-22) with anti-K14 specificity. The second group contains BRIC 107 and two further anti-k-like monoclonal antibodies (BS45 and OSK5). The results suggest that the polymorphisms encoded at the K/k and Kpa/Kpb/Kpc loci may be located in two spatially distinct regions of the Kell glycoprotein(s).