Abelson murine leukemia virus (A-MuLV) is derived from the thymotropic Moloney leukemia virus. However, injection of mice with A-MuLV by conventional routes results in rapidly arising peripheral and bone marrow lymphomas, not thymomas or T cell tumors. In this study thymomas have been induced by intrathymic injection of A-MuLV into BALB/c and C57BL/Ka mice. In both strains thymomas arose with short latent periods, comparable with the latencies of nonthymic tumors induced by intraperitoneal injection of A-MuLV and significantly shorter than those of thymomas induced by intrathymic injection of Moloney leukemia virus. Cells of the BALB/c thymomas were predominantly Thy-1-; those of C57BL/Ka thymomas were predominantly Thy-1+. Tissue culture lines were established and cloned. Some of these expressed low amounts of Thy-1 and one also expressed Lyt-1. Virus from cloned lines transformed 3T3 cells in vitro and induced Abelson disease in vivo when injected intraperitoneally into neonates. The A-MuLV p120 protein has been precipitated from metabolically labeled cell lysates of one cloned Thy-1+ line. These results show that A-MuLV can transform cells in the T lymphocyte lineage.