A new in vitro colony growth assay system method was found to be reliable in its use for evaluating B-cell proliferation in normal subjects and in 9 patients with B-cell chronic lymphocytic leukemia (B-CLL). The method is based on a phytohemagglutinin (PHA)-stimulated monocyte and T-cell-conditioned medium (PHA-MTCM) composed of PHA, silica, normal monocytes and normal T cells. The number of colonies proliferated was significantly greater in 5 patients who had not undergone treatment than in normal subjects (1,417 +/- 660 vs. 661 +/- 119) (p less than 0.002). Normal cultured B-cell colonies were shown to be 71% surface IgM colonies, and 6% cytoplasmic IgA colonies with the appearance of blastic cells. B-CLL colonies, on the other hand, were demonstrated to be monoclonal with the same CLL circulating cells being retained. We also studied the effect of interleukin-2 (IL-2) on B-cell colony growth assay in 4 patients with B-CLL cells. Only 1 patient with M protein responded to IL-2, proliferated and expressed IL-2 receptors. Although 3 patients without M protein did not respond to IL-2, they did respond to the supernatant, and they proliferated but expressed no IL-2 receptors.