The aberrant expression of antigens (Ag) in lymphoproliferative disorders may cause a diagnostic problem when single parameter immunohistochemical assays are performed on frozen or paraffin sections because coexpression by relevant cells is not determined. This aberrant expression also raises the question as to whether mixed lineage (biphenotypic) lymphoid proliferations exist. Marrow (6) and extramedullary (20) tissues from 26 patients with diffuse, intermediate and high grade, B-cell lymphomas (IWF E=1, F=1, G=19, H=1 and J=4) were analyzed with 19 markers using 3-color flow cytometry. The percentages (%) of patients with double Ag coexpression in at least 20% of the CD19+ or CD20+ lymphoma cells were: stem cell (SC) Ag: CD10 = 58 and CD34 = 15; T-cell Ag: CD2 = 38, CD5 = 19 and CD7 = 19; myeloid (My) Ag: CD13 = 19 and CD33 = 8. The corresponding % with unusual triple Ag coexpression in at least 10% of the CD19+ B-cells were SC+T+ Ag: CD10CD2 = 50, CD10CD5 = 27, CD10CD7 = 38, CD34CD2 = 31, CD34CD5 = 19 and CD34CD7 = 27; T+T+ Ag: CD2CD5 = 35, CD2CD7 = 42 and CD5CD7 = 31; T+My+ Ag: CD2CD13 = 35 and CD2CD33 = 12; and My+My+ Ag: CD13CD33 = 12. Ten of 12 lymphomas tested showed clonal immunoglobulin (Ig) heavy chain gene rearrangements in the absence of clonal T-cell receptor (TCR) gene rearrangements. None (0%) of the My Ag positive cases showed immunoreactivity for myeloperoxidase. We conclude that the anomalous T and My Ag expression seen in the above B-cell lymphomas is not indicative of mixed lineage proliferation but represents the aberrant expression of these antigens by the malignant cells.