Summary.In attempts to produce stable lines secreting bovine monoclonal antibodies, murine/bovine hybridomas (1° xenohybridomas) were selectively cultured in 8‐azaguanine to derive HAT‐sensitive lines that were then used as myeloma partners for further fusions with bovine lymphocytes. The resulting 2° xenohybridomas were further selected to produce 3° xenohybridomas. Four stable lines secreting bovine monoclonal antibodies recognizing blood group determinants X1(an IgG1), E‘2(an IgM) and SU'' (an IgGI) and another (an IgGI) as yet unidentified were produced from fusions of 2° xenohybridomas with lymphocytes from calves that had been immunized with bovine