A subpopulation of human B lymphocytes forms rosettes with mouse erythrocytes through a glycoprotein-dependent bond. Further studies of this bond show that the lymphocyte receptor is not immunoglobulin, although the binding of antisera or staphylococci to surface immunoglobulin inhibits the formation of mouse rosettes. Rosette formation could not be induced in thymocytes by enzymatic modification of the surface, or in T cells by lectin-induced transformation. The capacity to bind mouse erythrocytes was lost after incubation in a serum-free medium of lymphcytes from most patients with chronic lymphocytic leukaemia (CLL) and normal subjects. This loss could be prevented by the addition of a variety of sera and glycoprotein-containing substances to the medium, including fetuin. Conditions conductive to the subsequent restoration of rosetting capacity could not be found, indicating that the loss of this capacity was not due to the shedding of a cell-surface receptor which could be re-synthesized. It is suggested that the functional receptors in B1 lymphocytes are held in aggregates by cross-linking peripheral glycoprotein molecules; disaggregation and consequent loss of the capacity to form rosettes with mouse erythrocytes occurs during incubation in serum-free media, and during maturation of lymphocytes to the B2 stage.