Both retinoic acid (RA) treatment and dominant-negative c-Jun mutant expression effectively inhibit phorbol ester-induced AP-1 activity and induced neoplastic transformation in mouse epidermal JB6 cells. However, both reagents also target non-AP-1 molecules in addition. Because liganded retinoic acid receptors interact with and transactivate RA response elements (RAREs) on DNA, as well as interact with Jun protein to block AP-1 activity, the question arises as to which of these two activities of retinoids is responsible for antitumor-promoting activity. To address this question we generated JB6 promotion-sensitive (P+) cell lines that are stably transfected with a construct containing the collagenase promoter bearing one AP-1-binding site that drives a luciferase reporter gene. The stable collagenase-luciferase-transfected cell lines showed 1.5–3.5-fold enhanced AP-1 activity when treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Up to 90% of TPA-induced AP-1 activity was blocked by retinoids SR11238, SR11302, or trans-RA, but not by retinoid SR11235. Of these retinoids, only RA and SR11235 were able to transactivate RARE-dependent gene expression. Transrepression of TPA-induced AP-1 and transactivation of RARE by RA, SR11238, and SR11302 were concentration dependent at 10-10 to 10-6 m retinoid. When tested for activity in inhibiting tumor promoter-induced transformation in JB6 P+ cells, the retinoids specific for AP-1 transrepression were inhibitory, whereas SR11235, which only activated RARE, showed little effect. We thus conclude that the AP-1-blocking activity of retinoids is likely to be responsible for the antitumor-promoting activity. This result, together with the observation that dominant-negative Jun blocks transformation, argues for a requirement of induced AP-1 in the tumor promoter-induced transformation process.