Mouse erythrocyte-rosette-forming cells (MRFC) were determined in 76 cases of chronic lymphocytic leukaemia (CLL), 19 cases of centroblastic-centrocytic follicular non-Hodgkin's lymphoma (NHL), five cases of hairy cell leukaemia (HCL) and in 62 control patients. The mean percentage of MRFC in B-CLL was 54.8% +/- 24.4%, in nodular centro-follicular NHL 2.3% +/- 3%, in hairy cell leukaemia 14.6% +/-14.7% and among the controls 3.2% +/- 1.9%. Thus, MRFC appear to be an excellent marker of B-CLL and sometimes the only B cell marker as in 11 cases of SIg negative CLL. There is no correlation between the percentage of MRFC and clinical staging according to Rai classification whereas patients with stage 2 in Binet's classification (isolated splenomegaly) have a significantly lower MRFC percentage (23.4% +/- 27.7%; P less than 0.05). There is no correlation with prognosis, nor with serum Ig levels. We found no correlation with surface phenotype IgM, IgM-IgD or IgG. Thus, this study does not confirm the hypothesis that MRFC might characterize and immature stage in the B lymphocyte differentiation. However, these results are consistent with another hypothesis which states that lymphocytes in CLL possibly originate from a clonal expansion of a normal B lymphocyte subset, characterized by a low concentration of SIg and a receptor for mouse erythrocytes.