In lymphoma patients, treatment with chimeric CD20 monoclonal antibodies (rituximab) results in a depletion of normal and malignant B cells, persisting for 6 to 9 months. This B-cell depletion leads neither to a decrease in immunoglobulin levels nor an increase in the number of infectious complications. However, the effect of rituximab treatment on the immune responsiveness is unknown. In 11 patients with relapsed, low-grade lymphoma, we investigated the effect of rituximab treatment on the humoral immune response to 2 primary antigens and 2 recall antigens. After rituximab treatment, the humoral immune response to the recall antigens was significantly decreased when compared with the response before treatment. Already before rituximab treatment, none of these patients was able to mount a response to the primary antigens. These findings are relevant regarding the feasibility of rituximab in maintenance treatment and may also offer a rationale for the treatment of antibody-mediated autoimmune diseases with rituximab.