The appearance on single leukemic blast cells of markers of at least two different lineages has been termed lineage infidelity. MOLT-3 cells, a continuous line of human T lymphoblasts, express T cell markers as defined immunologically with monoclonal antibodies. Following a single exposure to 5-azacytidine, other markers, usually associated with non-T cell lineages, appeared transiently. A stable clone with lineage infidelity was obtained by selection from colonies expressing novel markers. Marker expression followed reproducible kinetics during growth. 5-Azacytidine-treated MOLT-3 subclones may be useful models in the study of lineage infidelity and gene expression.