Monoclonal antibodies reactive with hairy cell leukemia were developed to aid in the diagnosis of this subtype of B cell chronic lymphocytic leukemia and to gain better insight into the origin of hairy cells. Three antibodies were found to be of value in the diagnosis of hairy cell leukemia. Antibody B-ly 2 can be considered a pan-B cell reagent and generally reacts similar to CD22 antibodies. Antibody B-ly 6 is reactive with the same antigen as CD11c (p150/95), an antigen that is present on hairy cell leukemia, macrophages, and a minor subpopulation of lymphocytes. Antibody B-ly 7 is a unique antibody reactive with 144 Kd antigen present only on hairy cell leukemia and a very small population of normal B lymphocytes. This subpopulation may be the counterpart of hairy cells.