The mitogenic response of blood lymphocytes from twelve patients with B-cell CLL and seven healthy controls was investigated. All patients were untreated and had a typical B-cell dominance in their peripheral blood. The patients demonstrated a scattered response pattern with a higher variance than in the control group. We demonstrated examples of stimulation of DNA-synthesis by polyclonal B-cell activators (PBA) as well as inhibition. The variable pattern between the patients might result from a malignant transformation of cells originating from different lymphocyte subpopulations in different patients. Thus PBA studies of lymphocytes from CLL patients might be a valuable tool in characterizing further this clinically variable disease at the cellular level.