The JNKs (c-Jun N-terminal kinases) are stress-activated serine/threonine kinases that can regulate both cell death and cell proliferation. We have developed a cell system to control JNK re-expression at physiological levels in JNK1/2-null MEFs (murine embryonic fibroblasts). JNK re-expression restored basal and stress-activated phosphorylation of the c-Jun transcription factor and attenuated cellular proliferation with increased cells in G1/S-phase of the cell cycle. To explore JNK actions to regulate cell proliferation, we evaluated a role for the cytosolic protein, STMN (stathmin)/Op18 (oncoprotein 18). STMN, up-regulated in a range of cancer types, plays a crucial role in the control of cell division through its regulation of microtubule dynamics of the mitotic spindle. In JNK1/2-null or c-Jun-null MEFs or cells treated with c-Jun siRNA (small interfering RNA), STMN levels were significantly increased. Furthermore, a requirement for JNK/cJun signalling was demonstrated by expression of wild-type c-Jun, but not a phosphorylation-defective c-Jun mutant, being sufficient to down-regulate STMN. Critically, shRNA (small hairpin RNA)-directed STMN down-regulation in JNK1/2-null MEFs attenuated proliferation. Thus JNK/c-Jun regulation of STMN levels provides a novel pathway in regulation of cell proliferation with important implications for understanding the actions of JNK as a physiological regulator of the cell cycle and tumour suppressor protein.