The phenotype of fresh and cultured leukemic cells from patients with hairy cell leukemia was studied using a panel of monoclonal antibodies in addition to the detection of peroxidase activity under electron microscopy. In fresh samples, the leukemic cells from 11 patients displayed predominantly a B phenotype, as judged by their reactivity with the B1 monoclonal antibody and surface immunoglobulin expression. Ultrastructural peroxidase activity, characteristic of hairy cells, was observed in all cases studied. When hairy cells were cultured in the presence of phytohemagglutinin and irradiated T cells, their phenotype converted from surface Ig+, B1+, OKT3-, OKT11- to surface Ig-, B1+, OKT3-, OKT11+. In contrast, the peroxidase activity remained unchanged. Some hairy cells were also OKM1+, but no conclusion could be made about the MO2 antigen, a more specific marker of monocytes. The variability of the phenotype in vivo and in vitro indicates that reliable markers are required for identifying hairy cells. When studied together, the staining by B1 monoclonal antibody and the ultrastructural detection of peroxidase, enable the identification of hairy cells with certainty.