Murine CD4+ Th1 clones require de novo synthesis of proteins to express a cytotoxicity that is mediated by de novo synthesized Fas ligand (FasL). The cytotoxic process of the CD4+ Th1 effectors can be separated into four stages, namely conjugate formation, activation, lethal hit, and effector-independent target cell death. The present study describes the cytotoxic process in terms of FasL induction and Fas/FasL molecular interactions for death signal transduction. Fas-Ig fusion proteins, cycloheximide, actinomycin D, and EGTA+MgCl2 were used to analyze each stage of the cytotoxic process in terms of FasL/Fas participation. The results demonstrate that the activation-induced de novo mRNA and protein synthesis were for FasL, which provided the predominant cytotoxic activity of CD4+ Th1 effectors. Once activated, Th1 effectors express cytotoxic activity in the presence of EGTA+MgCl2, an experimental [Ca2+]ext-independent condition characteristic of FasL-mediated cytotoxicity. The ability of Fas-Ig to inhibit target lysis declined rapidly after conjugate formation, indicating that FasL-mediated lethal hit is critically dependent on conjugate formation and, once delivered, the effector-independent target lysis proceeds. After the lethal hit stage, transduction of Fas-mediated death signal was independent of de novo synthesis of macro-molecules in targets because treatments that inhibited more than 98% of the macromolecule synthesis had little effect on target lysis. Our study provides the first molecular view in terms of FasL/Fas of the cytotoxic process of CD+ Th1 cells.