Stable cloned lines of anti-B-secreting cells have been derived by fusing spleen cells of a mouse immunized by group B antigen with a mouse myeloma line. The tissue culture supernatant of one of them (NB1/19.112.28) containing secreted monoclonal anti-B antibody is an especially good blood-grouping reagent, well suited for use with manual and automated methods. The unlimited availability of a potent reagent of uniform properties, cheaply produced by a single source, makes monoclonal anti-B a serious competitor to human typing serum.