The capacity of synthesizing and secreting Ig molecules was studied in 11 patients with B-cell chronic lymphocytic leukemia (B-CLL) whose cells expressed surface IgM, in 3 patients with surface IgG-bearing cells, and in 2 IgM prolymphocytic leukemias (IgM-PLL). Three types of mu chains were detected by SDS-polyacrylamide gel electrophoresis analysis of the endogenously labeled molecules isolated by specific immunoprecipitation. Two of them were isolated from the cell lysates and were identified as the membrane mu chain and the precursor of the secreted molecules, respectively. The latter also possibly contained precursors of the membrane molecules. The third type of molecule was detected only in the culture medium and was identified as secretory mu chain. Not all of the malignant clones possessed the three types of mu chains. Only 7/13 of the IgM-bearing malignant cell clones were capable of secretion, whereas the remaining synthesized the secretory mu chains but degraded them intracellularly. Two types of molecules (membrane and secreted) were found in the IgG-bearing CLL cells from three patients. In all of them, secretion was detected. Ultrastructural analysis demonstrated that cells from the secreting clones had the features of more mature lymphocytes than the cells from nonsecreting clones. These features were represented by a developed Golgi apparatus, various types of vesicles (smooth and coated), and strands of the rough endoplasmic reticulum. A certain heterogeneity of the degree of maturation of the cells was observed within these clones. The data are consistent with the hypothesis that CLL clones are heterogeneous and can be distinguished through the different degrees of maturation of their cell components.