Blood lymphocytes from thirteen patients with CLL were studied for surface-bound Ig (SIg), Fc receptors (EA rosettes), receptors for sheep erythrocytes (E rosettes) and receptors for Helix pomatia A haemagglutinin (HP), a carbohydrate-binding protein with specificity for N-acetyl-D-galactosamine and related sugars. Fluorescein-labelled HP binds to subpopulations of human peripheral blood lymphocytes (PBL) treated with neuraminidase. In normal peripheral blood, HP binds to the T-lymphocytes while the majority of the B cells bearing surface-bound immunoglobulin do not have receptors for HP. In untreated CLL, HP binds to 90-100 percent of the neuraminidase-treated PBL. Almost all of the SI-G-POSITIVE CELLS IN CLL patients also have receptors for HP. Two groups of patients were found: in one the total fraction of SIg+ cells was less than or equal to 50 percent and about 30 percent of these lost their Ig during incubation at 37 degrees C. No such loss of SIg was revealed in the remaining patients where total SIg+ fraction was approximately 70 percent. These patients usually had higher blood lymphocyte counts, probably reflecting a more advanced disease. CLL patients in remission with low numbers of leukaemic cells also had low numbers of blood lymphocytes carrying both SIg and HP-receptors. It is concluded that leukaemic cells carry both HP receptors and SIg. Testing of this combination therefore provides a valuable new tool for monitoring patients with CLL.