By a PCR-based screen for cDNA clones of protein kinases, we have isolated a cDNA clone encoding a novel protein kinase (referred to as EDPK) of 305 amino acids. EDPK has a catalytic domain of 271 amino acids that contains all conserved subdomains characteristic of the protein kinase family. Only short sequences are present at the N- and C-terminal ends outside the catalytic domain. EDPK expressed in Escherichia coli and in mammalian cells phosphorylated serine and threonine, but not tyrosine, residues in an exogenous substrate. The amino acid sequence similarity between EDPK and known serine/threonine kinases was less than 35%. Thus, the newly isolated protein kinase EDPK is a novel member of the serine/threonine kinase family. Northern blot analysis showed that the EDPK mRNA was highly expressed in various stages of mouse embryo development. The expression of the mRNA was also found in a variety of mouse adult tissues. These results suggest that EDPK plays a crucial role in intracellular signaling not only during mouse development but also in adult tissues.