Five patients with hairy cell leukemia (HCL) were studied. Peripheral blood leukocytes, rosette-forming cells (T) and non-T-cells were stained in immunofluorescence by a panel of monoclonal antibodies to investigate the phenotype of HCL cells (HCLC). In all patients HCLC showed B-lymphocyte phenotype, although they were not stained by antibodies reactive for monocytes, natural killer cells, or T-cells. However, in all instances the large majority of HCLC were unexpectedly stained with an antibody (anti-CD1a) usually detectable only in early thymocytes and on Langerhans cells. This finding was further confirmed by immunoelectron microscopy. This type of ambiguity in the lineage of HCL could imply that HCLC might arise from cells differentiated towards the B-cell lineage, still sustaining an early antigen of a different (T) lineage. These results, moreover, extend the range of the known distribution of the CD1a antigen, which could be useful in diagnosing HCL.