Injection of newborn mice with mixtures of wild-type moloney murine leukemia (Mo-MuLV) virus and other recombinant retroviruses harboring the myc oncogene alone or in combination with the H-ras oncogene resulted in a 100% incidence of lymphatic leukemias from which permanent cell lines could be established in vitro. These cells are immunoglobulin (Ig)-, Thy-1+BP- and CD8-CD4- indicating that they are early thymocytes. Such transformed pre-T lines lack retroviral myc and ras genes but occasionally possess proviral insertion near to their endogenous myc and pim genes. We show that both Ig heavy chain (Igh) and T cell receptor (TcR) genes are rearranged in most of these lines. In some cases, a primary recombination was followed by a secondary rearrangement at the same locus. We show that VT gamma genes can rearrange outside of their known cluster suggesting that TcR gamma diversification in such pre-T cells may be different to that in more mature T cells. Ig D-JH recombinations may precede TcR gene recombination in these early T cell lines, and some but not all express sterile Cmu transcripts. Some of these lines express surface heterodimers that appear composed of alpha and beta chains that can be immunoprecipitated with a monoclonal anti-T3 antibody but not with the anti-V beta 8 monoclonal antibody F23.1. This established pre-T cell line represents novel biological material for the dissection of T cell development and function analogous to A-MuLV transformed pre-B cells.