The AML1 gene, which encodes the DNA binding subunit of the heterodimeric transcription factor, PEBP2/CBF, is involved in several types of chromosomal translocations associated with human acute myeloid leukemia, and has been shown by gene targeting to be essential for the development of definitive hematopoiesis in the murine fetal liver. In addition, the gene is expressed abundantly in T lymphocytes and has been implicated in T cell specific gene expression. In the present study we examined the function of AML1 in T cell receptor (TCR)-mediated, Fas/Fas-ligand dependent apoptosis of a T hybridoma line, DO11.10. Several independent cell clones overexpressing the AML1 protein were isolated by transfecting AML1 cDNA into these cells. These clones possessed an increased level of PEBP2/CBF DNA binding activity and were found to be resistant to apoptosis induced by anti-CD3 antibody treatment. Northern blot analysis revealed that induction of the Fas-ligand transcript was markedly suppressed in the anti-CD3 treated clones. Instead, expression of IL-2 receptor alpha subunit (IL-2R alpha), which is a manifestation of proliferative TCR signaling, was induced. This was in contrast to the parental, anti-CD3 treated DO11.10 cells where induction of Fas-ligand but not of IL-2R alpha was observed. Resistance of the AML1 overexpressing cell clones to TCR-mediated apoptosis is most likely attributable to the lack of Fas-ligand induction, since simultaneous treatment with anti-CD3 and anti-Fas antibodies caused apoptosis of the clones. The overall results suggest that the AML1 protein may play a pivotal role in switching TCR signaling between apoptosis and cell proliferation in T lymphocytes.