Mantle cell lymphoma is one of the most difficult to treat of all the non-Hodgkin's lymphomas. CAMPATH-1H, a humanized monoclonal antibody against the CD52 antigen, has been used with some success in other lymphoproliferative diseases, especially chronic lymphocytic leukemia. This report demonstrates that tumor samples from patients with mantle cell lymphoma express the CD52 antigen and suggests that CAMPATH-1H should be studied in patients with mantle cell lymphoma.