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 an in vitro B cell colony assay to study the in vitro growth, 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 CD19 (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 of in vitro culture. 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 both in vivo and in vitro. In vitro growth potential, as measured by the B cell colony assay, was also heterogeneous with three subgroups defined as low growth (< 10 colonies), intermediate (10-100 colonies) and high growth (> 100 colonies). Furthermore a T cell conditioned medium was not found to be a requirement for in vitro colony growth in the majority of CLL B cells. In addition, we evaluated the potential correlation of B cell CD5 phenotype or B cell colony growth on standard clinical parameters.(ABSTRACT TRUNCATED AT 250 WORDS)