Human breast tumors that are initially responsive to tamoxifen (TAM) eventually relapse during treatment. Estrogen receptor (ER) expression and function are often preserved in these tumors, and clinical evidence suggests that this relapse may be related to TAM's known agonistic properties. ER can interact with the activator protein-1 (AP-1) transcription factor complex through protein-protein interactions that are independent of ER DNA binding and, in certain ER-positive cells, this may allow TAM to exert an agonist response on AP-1-regulated genes. We, therefore, assessed both AP-1 DNA binding and the known AP-1 activating enzyme, c-Jun NH2-terminal kinase (JNK), in a panel of 30 ER-positive primary human breast tumors with acquired TAM resistance, as compared to a matched panel of 27 untreated control ER-positive breast tumors and a separate control set of 14 primary tumors, which included 7 ER-positive tumors that were growth-arrested by 3 months of preoperative TAM. AP-1 DNA binding activity was measured from cryopreserved tumor extracts using a labeled oligonucleotide probe containing a consensus AP-1 response element by electrophoretic mobility shift assay. JNK was first extracted from the tumor lysates by incubation over a Sepharose-bound c-Jun(1-89) fusion protein, and its activity was then measured by chemiluminescent Western blot by detection of the phosphorylated product using a phospho-Jun(Ser-63)-specific primary antibody. The set of control ER-positive breast tumors growth arrested by TAM showed no significant difference from untreated control tumors in their AP-1 DNA binding and JNK activities. In contrast, there was a significant (P < 0.001) increase in mean AP-1 DNA binding activity for the panel of ER-positive TAM-resistant (TAM-R) tumors as compared to its matched control panel of untreated tumors. Mean JNK activity in the TAM-R tumors was also significantly higher than that found in the untreated tumors (P = 0.038). Overall, there was no significant correlation between JNK activity and AP-1 DNA binding; however, regression analysis showed that, for any given level of JNK activity, the TAM-R tumors possessed a 3.5-fold increase in AP-1 DNA binding activity as compared to the untreated tumors. These findings indicate that, when compared to untreated ER-positive primary breast tumors, TAM-R tumors demonstrate significantly increased levels of AP-1 DNA binding and JNK activity, consistent with experimental models suggesting that TAM-stimulated ER-positive tumor growth may be mediated by enhanced AP-1 transcriptional activity. These observations support the need for further evaluation of these markers in breast tumors as predictors of TAM resistance.