Mantle cell lymphoma (MCL) is a CD5+ non-Hodgkin's B-cell lymphoma characterized by the infiltration of intermediate sized B-cells into the mantle zones. Interaction between CD40L and CD40 is important for B cell proliferation and differentiation. CD40L stimulation can induce both growth arrest and proliferation of B cell lines according to their differentiation state. Previous reports examining the effect of stimulation via the CD40 cascade on ex vivo MCL cells have provided conflicting results. In this study, two MCL lines, SP49 and SP53, were examined for response to CD40L and/or IL-10. Co-cultivation with CD40L-expressing mouse L cells reduced the BrdU incorporation of SP49 and SP53 cells by half to one-third, while BrdU incorporation of control cell lines, including Ramos, BJAB and BALL-1, was not affected or increased. Anti-CD40L antibody blocked the CD40L inhibition of SP49 cell proliferation in a dose-dependent manner in the range from 0 to 20 ng/ml. IL-10 did not affect MCL cell proliferation in the presence or absence of CD40L-expressing cells, while Ramos proliferation was promoted by CD40L and IL-10. These results suggested the possibility that CD40L may also inhibit MCL proliferation in vivo.