We have cloned both human and murine complementary DNAs that are homologous to the Drosophila serine/threonine polo kinase and the recently cloned murine polo related kinase (PLK). Both the human and murine clones are about 2.1 kilobases with open reading frames of 1.8 kilobases, encoding proteins of 603 amino acids with a predicted size of 66 kilodaltons and an apparent size of 67 kilodaltons by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. During embryonic development of the mouse, the mRNA was expressed in all tissues examined, whereas in adult tissues, expression was limited to thymus and ovaries. All cell lines examined also expressed mRNAs of similar size. Microinjection of in vitro transcribed sense mRNA into serum-starved murine NIH3T3 cells induced tritiated thymidine incorporation, whereas microinjection of antisense RNA into growing NIH3T3 cells blocked tritiated thymidine incorporation. When PC12 rat cells were induced to differentiate with nerve growth factor, gene expression of PLK was greatly reduced. Together, these results suggest that PLK expression is restricted to, and is perhaps required by, proliferating cells.