Chronic lymphocytic leukemia (CLL), despite an overall good prognosis, has a subgroup of patients with more rapid, aggressive disease. In an attempt to generate additional information about the B cell clones in B-CLL which could be used as predictive parameters, we analysed CD5 membrane phenotype and B cell colony growth in 29 B-CLL patients. CD5, a 67-kd glycoprotein, has been reported to be a consistent feature of the malignant B cell membrane phenotype in CLL. We used anin vitroB cell colony assay to study thein vitrogrowth, differentiation, and cell surface properties of CLL B cells. Finally, a variety of standard clinical parameters were collated for each patient. Monoclonal antibodies to both CD5 and CD 19 (pan B cell marker) were used to perform 2-color flow cytometry on freshly purified CLL B cells and on CLL B cells harvested after 7 days ofin vitroculture. We demonstrate here that CLL B cells are heterogeneous with respect to their expression of CD5, and that this expression is not fixed but may vary bothin vivoandin vitro. In vitrogrowth potential, as measured by the B cell colony assay, was also heterogeneous with three subgroups defined as low growth (85% CD5 positive) and lymphocyte doubling time less than 12 months and treatment within six months was detected. This study emphasizes that additional information regarding the biology of the malignant CLL B cell will generate new parameters that can be used to help predict the clinical course of B-CLL.