SCL (TAL-1) is implicated in the generation of human T-cell acute lymphoblastic leukaemia. To directly examine the role of this putative oncogene, an SCL retrovirus was constructed and used to infect a v-ABL transformed T-lymphocyte cell line. Thirteen independent SCL-infected and four control cell lines were established and injected subcutaneously into syngeneic mice. Mice injected with SCL-infected clonal cell lines died significantly more rapidly than control animals. By day 200 46% (40/87) of animals injected with SCL-infected cell lines had died due to disseminated transplantable lymphoid tumours. In contrast only 22% of control mice were dead by day 200 (P < 0.0015). Of possible relevance to the enhanced tumourigenesis, some SCL-infected cell lines displayed increased clonogenicity in agar. Increased cell growth was even more striking when ex-vivo tumour-derived cell lines were studied. Thus, SCL can co-operate with v-ABL to hasten T-cell tumourigenesis. This is the first direct evidence demonstrating that SCL can behave as an oncogene.