The oncogenic transforming potential of the jun oncogene was investigated with viral constructs that contain various terminal deletions of v-jun, c-jun and recombinants between the viral and cellular genes. Cellular jun can induce transformation of chicken embryo fibroblasts with a low efficiency. High efficiency transformation is dependent on alterations in the major transactivator domain. A jun deletion that lacks the transactivator domain is non-transforming. These observations are compatible with the conclusion that transcriptional regulation plays an important part in jun oncogenesis. In cells transformed by viral jun and expressing high levels of viral jun expression of the cellular jun is not constitutively upregulated. Cellular jun may therefore not contribute to transformation in these cells.