A simple reproducible in vitro B-cell colony assay system was used to evaluate B-cell growth in controls and patients with chronic lymphocytic leukemia (CLL). All six CLL patients studied formed B-cell colonies. The number of colonies was significantly less in patients than controls (66 +/- 18 versus 127 +/- 8). CLL colonies were shown to be monoclonal and appeared to reflect the circulating malignant B-cell clone in our patient group, while the six controls studied formed polyclonal B-cell colonies. Wright-Giemsa staining showed typical plasma cells to have developed in the controls but not in the patients. Cells from CLL patients retained a more lymphoid appearance. It is believed that investigations with this B-cell assay will provide the means for further in vitro evaluation of malignant B-cell proliferation in other lymphoproliferative disorders.