AbstractErythrocyte (E) rosette‐forming cells have been investigated in three patients with B chronic lymphocytic leukemia whose leukemic lymphocytes were easily identifiable. A small percentage of fresh neoplastic cells formed E rosettes in two patients. In every patient, most unstimulated, cultured leukemic lymphocytes became E+and this was further enhanced by phytohemagglutinin and pokeweed mitogen stimulation or neuraminidase treatment. These E+B cells lacked detectable T cell antigens (except for a weak expression of an antigen associated with the helper T cell subpopulation in one case). They were unreactive or weakly reactive by immunofluorescence with a monoclonal antibody to the E receptor. However, this antibody completely inhibited E‐rosette formation. The enhanced expression of E‐rosette receptors byin vitrocultured cells appeared to be dependent upon the presence of a small number of E‐rosetting cells at the beginning of the culture. E‐rosette receptor expression by leukemic lymphocytes was most likely in a fourth case (out of 9 patients studied). This finding may account for some of the discrepancies in the study of so‐called T cells in B chronic lymphocyt