Circulating malignant cells from 15 patients with B-cell chronic lymphocytic leukemia (B-CLL) were examined by the electron microscope (EM) and tested for their capacity to produce immunoglobulin (Ig) molecules using endogenous labeling techniques in vitro. In agreement with previous observations, B-CLL clones from various patients could be subdivided in three distinct groups: immature (type 1) clones, that comprised mainly small resting lymphocytes which synthesized, but degraded Ig of the secretory type intracellularly; mature (type 3) clones consisting mainly of cells with an extended Golgi apparatus and numerous strands of rough endoplasmic reticulum (RER) that secreted Ig molecules; and clones (type 2) at an intermediate maturational stage. When stimulated with T-cell supernatants that contained T-cell replacing factor(s) (TRF), type 3 and, to a lesser extent, type 2 clones differentiated further and secreted increased amounts of Ig. This maturation was not observed when type 1 clones were stimulated with the same supernatants. However, these cells were not incapable of further maturation since they could differentiate in response to phorbol ester (TPA). The present data reinforce the concept that different CLL clones undergo a process of maturation in vivo that may be arrested at different levels and demonstrate that the various clones respond differently to physiological stimuli depending upon the level of maturation already reached in vivo.