Jun/JunD and Jun/GCN4 chimeras transform chicken embryo fibroblasts and activate the collagenase promoter in these same cells. Individual constructs differ widely in the two activities, and there is no correlation between transformation and transactivation. These results suggest that oncogenic transformation by Jun is not caused merely by an upregulation of AP-1 activity. Jun constructs with a modified dimerization domain allowing only homodimerization are active in transformation and transactivation in chicken embryo fibroblast cultures. Homodimers of Jun therefore transform and transactivate.