Mantle cell lymphoma (MCL) is a B cell non-Hodgkin's lymphoma, characterized by a poor response to therapy and short survival. To assess the proliferative capacity, we cultured MCL cells, using irradiated 3T6 mouse fibroblasts transfected with human CD40L (‘CD40 system’) in the presence of different cytokines. Proliferation was measured by 3 H-thymidine incorporation and by CFSE fluorescence. Thirteen out of 16 MCL cases proliferated well in the CD40 system. In 10 cases a strong response upon further addition of IL-10 was seen, whereas IL-4 had an additional effect in only four cases. CFSE staining of cells before and after culture showed an increased number of cell divisions in the IL-10/CD40L stimulated cells. The MCL cells remained CD5+ CD19+. Neither plasma cell differentiation nor isotype switching was seen. The light chain expression was strictly monoclonal. IL-1β, IL-2, IL-6, G-CSF and GM-CSF did not stimulate MCL proliferation. IL-10 receptor expression correlated with the response to IL-10 in the culture system and the effect of added IL-10 could be blocked by antibodies directed against IL-10 and the IL-10 receptor. Autocrine IL-10 production by the MCL cells was detected in eight of 10 cases tested. IL-10 receptor blocking decreased proliferation when no exogenous IL-10 was used in four of seven cases tested. EBV assessed by EBER in situ hybridization was not detected in six cases tested. In conclusion, MCL can successfully be cultured upon CD40 stimulation if 3T6 CD40L+ cells are used. In this context IL-10 is a costimulatory factor. IL-10 receptor expression seems to correlate with response to CD40 crosslinking and IL-10. Autocrine IL-10 production might play a role in the proliferation of this lymphoma. This culture system may be useful to test new treatment strategies for this, thus far, therapy-resistant lymphoma.