We used a new antibody (NCL-CD5-4C7) immunoreactive for CD5-positive cells in paraffin-embedded tissue, with the microwave antigen retrieval method, to analyze a number of well-characterized cases of B-cell and T-cell non-Hodgkin's lymphomas whose CD5 immunoreactivity was documented by immunohistochemical analysis performed on frozen sections and/or flow cytometric immunophenotypic analysis of neoplastic cell suspensions. In all of the cases included in the study, small, reactive T lymphocytes within histologic sections were strongly positive for CD5 and served as an internal positive control. All CD5-positive T-cell non-Hodgkin's lymphomas (13 [100%] of 13) from several histologic subgroups, were immunoreactive for CD5 in paraffin sections, although 4 of 13 exhibited weak or limited staining of neoplastic cells. The majority of cases of B-cell small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL) (13 [81%] of 16) and mantle cell lymphoma (14 [93%] of 15), both B-cell lymphoproliferative disorders that are immunoreactive for CD5, were immunoreactive for CD5 in paraffin sections, although 5 of 13 cases of SLL/CLL and 6 of 14 cases of mantle cell lymphoma exhibited weak and/or limited staining of neoplastic cells. Other B-cell non-Hodgkin's lymphomas of various subtypes, including marginal zone, follicular, diffuse large cell, and lymphoblastic types, were nonreactive for CD5 in paraffin sections, as expected. CD5 antibody NCL-CD5-4C7 should be useful for the routine characterization of suspected T-cell and B-cell lymphoproliferative disorders when only paraffin-embedded tissue is available for immunophenotypic analysis.