Many low grade B-cell lymphomas, and most T-cells lymphomas, express CD5 on their surface. This expression has been demonstrated in fresh cells or frozen sections. Recently, a new monoclonal antibody to CD5, NCL-CD5, has been introduced that detects CD5 in paraffin-embedded tissues. Paraffin-embedded tissues from five patients with small lymphocytic lymphoma (SL), four patients with chronic lymphocytic leukemia (CLL), and one patient each with large granular lymphocytic leukemia, diffuse large cell lymphoma (T cell), and mycosis fungoides were stained with the NCL-CD5 antibody after unmasking the antibody with the steam/citrate technique. All these cases had CD5 positivity demonstrated by flow cytometry or on cytospin or frozen section preparations. In addition, one case of angiofollicular lymph node hyperplasia (CD5+), two cases of SL/CLL (CD5-), and one case CD5- T-cell lymphoma were also investigated. Of the 12 CD5+ malignancies, only 1, an SL, was positive with the NCL-CD5 antibody. In seven of these cases, both B-5 and formalin-fixed tissues were tested; the one positive case was positive only in the formalin tissue. The three CD5- malignancies were also negative in paraffin sections (P = .001; McNemar test). However, reactive T cells did stain in these biopsy sections. The case of Castleman's disease showed many CD5+ cells with the NCL-CD5 antibody. Although NCL-CD5 does indeed stain reactive T cells in paraffin sections, it does not appear to stain neoplastic CD5+ cells.