Cells from the blood of patients with chronic lymphocytic leukaemia were cultured in the presence of two polyclonal activators of human B cells, the bacteria Branhamella catarrhalis (Bc) and Staphylococcus aureus Cowan 1 (SAC). Although the magnitude of the responses varied, cells from seven of the eight patients studied were induced to proliferate in response to Bc. In contrast, the response to SAC was low or negligible in seven of the eight patients, and only one patient responded well to this mitogen. Bc was also effective in inducing secretion of IgM in cells from seven of the eight patients, and this was unaffected by removal of T cells. Fractionation of CLL cells on density gradients showed that the highest level of IgM production was induced in cells with a low buoyant density, whilst cells with a high buoyant density secreted little or no immunoglobulin in response to Bc. Together, these results demonstrate that Bc is an effective, T-independent activator of both DNA synthesis and immunoglobulin production in CLL cells.