The surface marker phenotype of lymphocytes derived from 12 patients with B-CLL was compared to that of lymphocytes from 10 patients with an other monoclonal but clinical benign form of B-cell proliferative disorder termed monoclonal B-cell lymphocytosis of undetermined significance (B-MLUS). A panel of well characterized monoclonal antibodies was used for the surface marker determinations. The mean total number of B cells (CD20) was 8.5 x 10(9)/1 in B-MLUS as compared to 44 x 10(9)/1 in B-CLL (p less than 0.001). B-CLL had a greater imbalance in T-cell subpopulations than B-MLUS and healthy controls. Total numbers of CD3+, CD8+ cells as well as cells expressing the NK-related antigens (CD16, Leu-7) and IL-2 receptor (CD25) bearing lymphocytes were statistically significant higher in B-CLL than in B-MLUS. Analyses of B-cell enriched populations showed that B-CLL represented B cells of an early maturation stage, whereas B cells from B-MLUS were more mature as judged by the loss of the CD21 surface marker. A larger fraction of B cells in B-CLL compared to B-MLUS exhibited a higher activation stage as revealed by the expression of the CD21, CD25 and CD35 structures as well as the FMC7 antigen.