The densities of surface immunoglobulin (slg) on peripheral blood mononuclear cells (PBM) of normals and patients with chronic lymphocytic leukemia (CLL), chronic lymphosarcoma cell leukemia (LCL), and hairy cell leukemia (HCL) were analyzed using the fluorescence-activated cell sorter (FACS). PBM were labeled with fluorescein conjugates of F(ab')2 fragments of affinity chromatography-purified anti-Fab or class-specific anti-mu, anti-delta, anti-gamma, or anti-alpha. Histograms of relative cell fluorescence, rems of relative cell fluorescence, reflecting slg density, were prepared with the FACS. Anti-Fab-labeled normal PBM demonstrated a homogeneous low-density peak that when separated by the FACS and analyzed cytochemically consisted predominantly of monocytes, whereas brighter-staining cells were predominantly lymphocytes. Anti-mu and anti-delta labeled 9.0% and 8.5% of normal PBM, respectively, the slg+ cells being virtually all lymphocytes. Cells labeled by anti-gamma exhibited low homogeneous slg density and consisted of more than 80% monocytes. No normal or leukemic PBM were labeled by anti-alpha. All slg-positive cells (less than 5% monocytes) from 12 of 13 patients with CLL had very low homogeneous densities of slg and bore slgM, Whereas cells from 9 of 13 and 2 of 13 patients bore slgD and slgG, respectively. Similarly, PBM from 2 patients with HCL exhibited low and homogeneous densities of algM, slgD, and slgG, whereas those from a third patient bore only slgG. By contrast, the density of slgM and PBM derived from 3 patients with LCL was very high; slgD and slgG densities varied from very high to undetectable in these patients. The different homogeneous densities of slg on peripheral blood lymphocytes from patients with CLL, HCL, and LCL suggest that these diseases represent malignant transformation of different B-lymphocyte subpopulations.