AbstractA human‐mouse heterohybridoma (HMR0921) secreting human monoclonal IgG3, Λ antibody was produced from peripheral blood lymphocytes of a healthy blood donor with serum antibody to Jra, by EBV transformation and hybridization with mouse myeloma cell line P3X63Ag8.653. The reactivity of HMR0921 antibody was assessed by antiglobulin test with a panel of red cells including 14 different rare blood types. Only Jr(a‐) red cells were negative. The strict specificity of this antibody to Jraantigen was further confirmed by absorption test with fluorescence flow cytometry. On screening of 28,744 blood donor samples by HMR0921 antibody, we detected 19 agglutination‐negative samples, which were confirmed as Jr(a‐) by conventional anti‐Jraantisera. Therefore, our HMR0921 antibody is extremely useful for detecting rare Jr