Fas ligand (FasL) produced by activated T cells mediates autocrine- induced apoptosis to limit T cell expansion. To investigate the regulation of FasL activity, Jurkat cells were stably transfected with a 2.3-kb fragment of human FasL promoter that controlled the expression of a GFP reporter gene. Two populations of Jurkat cells with different levels of GFP expression were obtained. One population constitutively expressed high levels of GFP (GFP+), while the other population expressed low levels of GFP (GFP-). The level of GFP expression in the two populations correlated with their levels of FasL transcription and its functional activity. Autocrine regulation of apoptosis was demonstrated by increased FasL activity after stimulation of GFP- cells with anti-CD3, phorbyl myristyl acetate plus ionomycin, or Con A. Paracrine regulation of apoptosis was suggested by the induction of apoptosis of GFP- cells after coculture with unstimulated GFP+ cells. GFP+ cells exhibited a decreased sensitivity to FasL-mediated apoptosis compared with GFP- cells. Furthermore, the cell surface expression of Fas and CD4 was lower on GFP+ cells than GFP- cells, whereas the expression of CD45RO was higher. A decreased level of IL-2 was produced by GFP+ cells after phorbyl myristyl acetate and ionomycin stimulation. Our results indicate that a subpopulation of T cells that express low levels of FasL and IL-2, which are responsive to up-regulation of these molecules after activation, can undergo apoptosis either by suicide after activation or by a paracrine pathway mediated by T cells that constitutively express higher levels of FasL.