Mouse erythrocytes form spontaneous rosettes with a population of B lymphocytes from normal individuals and from patients with B cell chronic lymphocytic leukaemia (CLL). Since lymphocytes from patients with CLL respond poorly to pokeweed mitogen (PWM), we have compared mouse rosette positive (MR+), mouse rosette negative (MR-), and unfractionated B lymphocytes from normal individuals in their response to PWM. Mononuclear cells were fractionated into B, MR+ and MR- cell populations and then combined in 1:1 proportions with mitomycin-C treated T cells in culture media. Lymphocyte co-cultures were incubated for up to 10 days in the presence of PWM. Supernatant immunoglobulin (SIg) levels, percentage intracytoplasmic immunoglobulin (ICIg), and proliferative responses were determined. MR+ cells alone failed to produce significant levels of SIg (P less than 0.025) or percentages of ICIg positive cells This decreased synthesis of immunoglobulin by MR+ cells occurred in the presence of adequate T cells, macrophages and a satisfactory proliferative response.