The myc oncogene is implicated here in T lymphocyte neoplasia. Cloning revealed a retroviral insert 0.7-1.3 kb 5' to c-myc in two T lymphomas induced by Soule murine leukemia virus and in a spontaneous T lymphoma ( Tikaut ) of an AKR mouse, a strain in which leukemogenesis involves recombinant retroviruses (MCF viruses). The tumor c-myc mRNAs appear normal but their level is approximately 5-fold higher than in most T lymphomas lacking c-myc rearrangement. Since each insert would be transcribed away from c-myc, its activation cannot involve the promoter of the long terminal repeat (LTR) but could reflect an enhancer, like that demonstrated within the Soule LTR. The Tikaut provirus has an MCF-like recombinant env gene and LTR sequence. MCF-like inserts were found near c-myc in seven of 31 other AKR T lymphomas; two lie 3' to c-myc and the five upstream are oriented away from c-myc. We conclude that a quarter of retrovirus-induced T lymphomas involve activation of c-myc, probably via the LTR enhancer.