Mononuclear cells from normal human subjects and patients with chronic lymphocytic leukemia (CLL), chronic lymphosarcoma cell leukemia (LCL), and hairy cell leukemia (HCL) were labeled with fluoresceinated, purified human C3b (FI-C3b) and analyzed using the fluorescence- activated cell sorter (FACS). FI-C3b labeled 17.6% +/- 6.0% of peripheral blood mononuclear cells (PBM) from 20 normal subjects, which, when separated by the FACS, consisted of B lymphocytes and approximately 5% monocytes. Analyses in which either monocytes or B lymphcoytes were excluded from consideration demonstrated that both these cell types were labeled by the FI-C3b with a heterogeneous distribution of fluorescence intensity, indicating either heterogeneity of CR density or variable avidity of individual CR for the FI-C3b. FACS profiles of PBM ( < 5% monocytes) from 14 of 15 patients with CLL showed a homogeneous distribution of very low fluorescence intensity, with > 60% of the cells being slightly more fluorescent than unlabeled controls. This low, homogeneous distribution of fluorescence is strikingly similar to profiles of CLL cells labeled with anti-Ig reagents and suggests homogeneity of low CR density and/or avidity. Similarly, CR+ mononuclear cells from five patients with HCL and three patients with LCL displayed more homogeneous FI-C3b labeling than normal CR+ PBM. Homogeneity of FI-C3b binding to CLL, LCL, and HCL cells further supports the concept for a clonal origin for these disorders.