A hairy cell leukemia (HCL) line, ZK-H, and a normal lymphoblastoid cell line, ZK-N, were established from the peripheral blood of a 69-year-old male patient. The ZK-H cells and the patient's original hairy cells shared the same surface properties; both possessed membrane-bound IgG with kappa light chains and villous surface structures. The ZK-N cells were devoid of villous surface structures and polyclonal in that they consisted of cells having different membrane-bound heavy and light chains. Both the ZK-H and ZK-N lines carried Epstein Barr virus (EBV)-determined nuclear antigen, but the patient's fresh leukemic cells lacked this antigen. The ZK-H line had a hyperdiploid chromosome constitution of 47 and trisomy No. 2, but the ZK-N line had a normal chromosome constitution. The presence of membrane-bound immunoglobulin and B-cell tropic EBV in the ZK-H line provides evidence of the B-cell nature of HCL in this patient.