The capacity for expression of Fas-Ligand (Fas-L) by naive CD4+ T cells is limited, but is rapidly acquired by antigen-stimulated Thl-type cells early in effector cell development. Fas-L expression by Th2-type cells is suppressed during effector development. Because essentially all activated Thl cells are sensitive to Fas-mediated apoptosis stimulated by anti-Fas antibody cross-linking, and yet only a limited fraction of antigen-activated cells die during a given round of antigenic stimulation in the absence of antibody, we postulated that Fas-L expression within this popualtion was limiting. Fas-L expression by single cells derived from aβ-TCR transgenic mice was analyzed during effector cell development using a novel quantitative, competitive RT-PCR method. A subpopulation of antigenactivated Thl effectors expressed Fas-L in comparison to IFN-y, that was expressed by twice as many cells. The copy number of Fas-L mRNA molecules per positive T cell was significantly less than that for IFN-y. These data indicate that Fas-L expression by the Thl population may be limiting, whereas all activated Thl cells express Fas in a functional form.