Immunotherapy with rituximab alone or in conjunction with chemotherapy has significantly improved the treatment outcome of B-cell lymphoma patients. Nevertheless, a subpopulation of patients does not respond to rituximab. The reason for treatment failure as well as the exact mechanism of action is still uncertain. The function of rituximab has long been associated with the partitioning of CD20 molecules to lipid microdomains. We now show that the extent of CD20 recruitment to lipid rafts correlates with response to rituximab. In addition, expression of the raft-associated sphingolipid GM1 on lymphoma cells is associated with the susceptibility of lymphoma cells to rituximab. Furthermore, we show substantially different GM1 expression in various primary non-Hodgkin's lymphomas. Whereas chronic lymphocytic leukemia (CLL) cells have a low GM1 expression, marginal zone lymphoma cells exhibit much higher levels. Differences were not only detected among various lymphoma subgroups but also within one lymphoma subtype. Interestingly, whereas CLL cells from patients with high GM1 expression responded to rituximab, patients with low GM1 expressing CLL cells did not. These data show the importance of membrane microdomains in the effect of rituximab and may offer a predictive factor for the responsiveness of lymphoma cells to rituximab.