The transcription factor v-Rel is a transforming protein of the reticuloendotheliosis virus. We found that v-Rel activates the promoter of the proto-oncogene c-jun. Two elements in the c-jun promoter were required for the activation by v-Rel. One was a kB-site (v-Rel binding site), and the other was a c-jun promoter region between -52 and +148 (c-jun promoter (-52/+148)). Two promoters with the kB-site(s), those of human immunodeficiency virus (HIV) and SV40, were not activated by v-Rel, but their kB-sites were activated when introduced upstream of the c-jun promoter (-52/+148). Thus, the c-jun promoter (-52/+148) had information for the selective activation of the c-jun promoter by v-Rel. v-Rel bound to the c-jun kB-site with the higher affinity than c-Rel, thereby activating the c-jun promoter more efficiently than c-Rel. Moreover, the activity of v-Rel mutants upon the c-jun promoter correlates with their transforming activity. Thus, the c-jun promoter activation by v-Rel may play a role in the transformation caused by v-Rel.