CD38 ligation in the presence of IL-2 triggers B-CLL proliferation. B-CLL cells purified from 12 different CD38⁺ patients were cultured for 3 days in the presence of the indicated agents. The simultaneous presence of an agonistic anti-CD38 mAb and of IL-2 increases ³H-thy incorporation. Analysis of purified B cells from 3 CD38^– patients (nos. 13, 16, and 28) indicates that CD38 is unable to deliver significant growth signals either alone or in association with IL-2. Error bars indicate SD.