Identified as an immediate-early transcript, the serum-inducible kinase Snk bears sequence homology with the polo-like kinases. Endogenous Snk was detected in early G(1) in NIH 3T3 cells, and nascent Snk showed a half-life of about 15 min. The kinase activity of endogenous Snk was detected in G(1). Substitution of Thr-236 with a glutamate residue increased Snk kinase activity by about 10-fold, whereas substitution of Lys-108 abolished its kinase activity. Disrupting the polo-box did not significantly change Snk kinase activity. A GFP-C-Snk fusion protein showed polo-box-dependent localization to the microtubule organizing center, and ectopic expression of Snk in COS-7 cells induced changes in cell morphology, depending on Snk kinase activity and the polo-box. The capacity of Snk to induce morphological changes was inhibited by the calcium- and integrin-binding protein CIB. CIB co-immunoprecipitated with Snk and inhibited the kinase activity of Snk, suggesting that CIB is a negative regulator for Snk kinase activity.