We report the simultaneous expression of T-cell antigens on the myeloma cells from six patients with multiple myeloma (MM). These six patients come from a total population of 215 samples (115 direct samples, clinical incidence of 5.2%) of plasmacytic malignancies immunotyped at the University of Arizona. Four patients expressed T helper antigen (Leu 3, CD4), one expressed T-cell antigen receptor (Leu 4, CD3), and one expressed E-rosette antigen receptor (Leu 5, CD2). The presenting clinical features, histology, and plasma cell morphology showed no differences from multiple myeloma patients who did not express T-cell antigen. However, although the survival duration ranged from 5 to 93 mo overall, survival from demonstration of T antigen expression was very short, (2 to 7+ mo), with five of six (80%) patients dying less than or equal to 5 mo after study. The reason for T antigen expression is unknown. It may indicate that myeloma can arise from a normally minor subpopulation of B cells involved with immunoregulation; conversely, it could be a coincidental aberrancy associated with malignant change in the plasma cells.