CD148 and CD27 are activation antigens involved in B cell and T cell activation and development. They have been recently proposed as markers of normal human memory B cells corresponding to the presence of somatically hypermutated IgV genes. We undertook an immunohistochemical study of CD148 and CD27 expression on neoplastic B cells in 116 cases of B cell non-Hodgkin's lymphoma. All cases of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), mantle cell lymphoma, Burkitt's lymphoma, and the vast majority of cases of marginal zone B cell lymphoma and most cases of plasmacytoma/myeloma expressed CD148 and CD27. Follicular lymphoma and diffuse large B cell lymphoma were also immunoreactive for CD148 and CD27 with some variation and discordance in expression. Cases of precursor B-lymphoblastic lymphoma/leukemia did not express CD148 or CD27. Our findings demonstrate that CD148 and CD27 are expressed in a wide range of B cell non-Hodgkin's lymphomas, and, therefore, do not serve to distinguish between neoplastic cells of naïve and memory B cell derivation.