PBMCs from patients with T-LGL leukemia do not manifestex vivospontaneous proliferation in a 6-dayex vivoculture. PBMCs from patients with T-LGL leukemia were placed in a 6-day culture in1640 media containing 10% FCS and proliferation was assessed by the evaluation of the uptake of thymidine added during the last 4 hours of culture. There was no meaningful thymidine uptake, indicating that there was essentially no ex vivo proliferation of PBMCs from the 6 patients studied with monoclonal T-LGL leukemia. When 20 000 pg/mL of IL-15 was added to the T-LGL of 5 patients, there was an increase in the geometric mean proliferation from 472-5 565 cpm per 1 millicurie of ³H thymidine added. This proliferation was inhibited to baseline levels of 728 cpm on addition of 10 μg/mL of Hu-Mikβ1 at the onset of the cultures.