A human CD4- CD8- alpha beta T-cell receptor (TCR alpha beta)+ T leukemic cell line, L-KAW, was established from the peripheral blood of a 6-year-old male patient with T-cell acute lymphoblastic leukemia. Its phenotype was found to be CD7+CD2+CD3+CD4-CD8- TCR alpha beta +. The cell line proved susceptible to killing by exposure to a mitogenic concentration of phytohemagglutinin (PHA) within several hours, with biochemical analyses demonstrating extensive degradation of DNA to oligonucleosomal bands characteristic of apoptosis. Chromatin condensation and cell shrinkage features of apoptotic cells could also be clearly identified under the electron microscope. The apoptosis-related Fas antigen was expressed on L-KAW cells and DNA fragmentation was induced by incubation with anti-Fas monoclonal antibody. The c-myc mRNA levels declined within 15 min to 3 h after the addition of PHA. These results suggest that L-KAW cells are specifically sensitive to induction of apoptosis in response to PHA without preactivation. They may, therefore, be considered an analog of activated T-cells and prove useful as a model system for characterizing biochemical and molecular mechanisms underlying the process of this type of cell death.