The newly produced monoclonal antibody (mAb) LF61 detects a molecule restricted to hairy cell leukaemia (HCL) among B-cell non-Hodgkin's lymphomas. In particular, the percentage of LF61 + HCL cells in different cases ranges from 10% to 100%. In normal lympho-haemopoietic tissues LF61 reacts with only about 2% of T-cells, mostly of the CD8 subset in the peripheral blood, extrafollicular areas of the tonsil, red pulp of the spleen and thymic medulla. Expression of the LF61 molecule is observed following stimulation of peripheral blood lymphocytes with phytohaemagglutinin (PHA) or pokeweed mitogen (PWM), suggesting that it represents an activation antigen. Due to its restricted reactivity with a small subset of normal CD8 + T-cells, LF61 in combination with a CD22 mAb is highly suitable for monitoring residual disease in interferon or deoxycoformicin-treated HCL patients. Polyacrylamide gel gradient electrophoresis shows that LF61 precipitates a 150 kDa, 125 kDa, 105 kDa trimeric molecule from the surface of HCL cells. Immunohistological and immunobiochemical results show that this molecule is the same as the one recognized by the still unclustered anti-HCL mAb B-ly7.