The functional specificity and T cell receptor (TCR) Vβ gene expression of three class II HLA-DR1-reactive human T cell populations were examined. WP2, a renal allograft-derived, long-term CD4+ T cell line, was specifically cytotoxic for DR1, one of the mismatched antigens present on the allograft. Initial studies of WP2 using six TCR Vβ-specific mAb revealed a predominance of T cells expressing a member of the β8 gene family. A smaller, yet significant, number of cells expressed TCR using Vβ5.1. Semiquantitative Vβ-specific polymerase chain reaction (PCR) analyses of RNA derived from this T cell line confirmed the presence of Vβ8 and Vβ5.1 messages. The PCR signal for Vβ8 was the strongest, followed by those for Vβ4 and Vβ5. An earlier WP2 culture had a very similar PCR profile, with a dominant signal for Vβ8, although signals for Vβ4 and Vβ5 were considerably lower. Previous PCR analyses of eight other renal allograft-derived, long-term T cell lines, grown under identical in vitro conditions, revealed no other example of predominant usage of Vβ8. We established two replicate long-term, anti-DRl mixed lymphocyte reactions using PBL from two unrelated normal donors as responder and stimulator. The MLR were given alloantigen every 10 days, and RNA was obtained from the cultured cells immediately prior to each stimulation. PCR analyses of RNA taken at 10-day intervals over a total of 60-70 days indicated that, although the MLR were initially quite heterogeneous with regard to Vβ message expression, by the end of the fourth or fifth Ag cycle the predominant PCR signals observed in both MLR were for V/88. These results suggest that T cells using Vβ8 gene-encoded segments as part of their TCR may have a selective advantage in responses to DR1.