Phosphorylation of histone H3 at serine 10 (S10) is essential for the onset of mitosis. Here, we show that basal c-Jun N-terminal kinases (JNKs) are required for mitotic histone H3-S10 phosphorylation in human primary fibroblast IMR90 cells. Inhibition of JNKs by specific pharmacologic inhibitors, expression of dominant-negative JNK1 and 2 mutants, or RNAi of JNK1 and 2 prevented phosphorylation of histone H3 at S10 in vivo. The JNK-specific inhibitor SP600125 blocked mitotic entry, as shown by its ability to prevent CDK1 dephosphorylation and cyclin A degradation. Basal JNK phosphorylation increased at G(2)/M phase, although total JNK protein levels remained unchanged. In addition, basal JNKs were localized in nuclei and centrosomes during this time, suggesting that the nuclear localization of JNKs during G(2)/M is tightly coupled with histone H3 phosphorylation. Basal JNKs were able to phosphorylate histone H3 in vitro and coprecipitation of histone H3 and JNKs was only detected at G(2)/M. Taken together, these data strongly suggest that basal JNKs play a key role in controlling histone H3 phosphorylation for mitotic entry at G(2)/M phase.