CD20 is a 35-kDa protein that is expressed early in B-cell ontogeny and is lost during terminal B-cell differentiation into plasma cells. It is thought to be B-cell-specific. However, the CD20 antigen, detected by the monoclonal antibody L26, has been reported in some cases of T-cell lymphoma. This report describes a case of a malignant lymphoma coexpressing T-cell-lineage antigens and CD20 and characterization of a CD20+ T-cell population in the peripheral blood of healthy donors. The tumor cells were pleomorphic medium-sized cells that expressed a range of T-cell-specific antigens, including CD2, CD3, CD4, CD5, CD6, CD7, and beta F1. In addition, the tumor cells expressed CD20 on frozen (B1) and paraffin sections (L-26). Stains for other pan-B cell antigens, including CD19 and CD22, and immunoglobulin light and heavy chains were negative. To determine whether this unusual coexpression of T-cell-lineage antigens and CD20 represented aberrant antigen expression or neoplastic transformation of an unusual normal T-cell subset, the authors examined specimens of peripheral blood lymphocytes from healthy donors for evidence of a CD20+ T-cell population by using three-color immunofluorescence analysis by flow cytometry. Two distinct populations of CD20+ cells were observed in peripheral blood. One expressed bright CD20 (6.6% to 23.7%, mean 14.47% of peripheral blood lymphocytes) and other B-cell associated antigens, whereas the other expressed dim CD20 (.94% to 11.90%, mean 3.50% of peripheral blood lymphocytes) and coexpressed CD3. Approximately two thirds (52.8% to 82.3%, mean 64.1%) of the dim CD20 cells were CD8+ and one third (19.2% to 74.1%, mean 37.5) CD4+. These cells also expressed CD5 and the alpha-beta chain of the T-cell receptor and lacked CD19 and CD22. These results indicate that CD20 is expressed on some normal peripheral blood T cells. CD20 expression by T-cell lymphomas may represent neoplastic transformation of a normal subset of CD20+ T cells rather than aberrant antigen expression by neoplastic cells. The nature of the CD20 antigen on T cells and the function of the normal population remain to be determined.