Ligation of CD40 is essential for primary B-cell activation and expansion and yet has suppressive or apoptotic effects on some CD40-expressing neoplasia. SGN-14 is a monoclonal antibody that binds to the human CD40 receptor. Here we report that SGN-14, in the presence of interleukin 4, provided a modest level of stimulation of peripheral blood B cells, as measured by proliferation. Stimulation was greatly enhanced in the presence of nonproliferating CD40 ligand-expressing cells. The enhanced agonistic activity could be attributed to a dose-dependent increase in CD40L binding to CD40 in the presence of SGN-14. In contrast to its proliferative effect on primary B cells, SGN-14 inhibited the growth of B-cell-derived tumor lines in vitro, and this growth inhibition was enhanced in the presence of CD40L-expressing cells. In vivo, SGN-14 showed significant antitumor activity in treating human B-cell lymphoma and multiple myeloma xenografted severe combined immunodeficient mice. Antitumor activity was not diminished by blunting murine natural killer activity, suggesting that CD40 ligation contributes to the antitumor efficacy of SGN-14. On the basis of these activities, SGN-14 is being pursued for therapeutic use in treating patients with CD40-expressing hematological malignancies.