Highly degenerate oligonucleotide primers designed from regions conserved between protein-serine kinases have been used specifically to amplify human epithelial (HeLa) cDNA by the polymerase chain reaction (PCR). Of several novel cDNA fragments encoding putative kinases thus isolated, one was further characterised. Screening of human fibroblast and bovine brain cDNA libraries with the PCR fragment yielded several clones with an open reading frame of 479 amino acids containing all of the conserved sequence motifs of protein-serine kinases. The predicted protein was most similar to the protein kinase C (PKC)/cAMP-dependent protein kinase (PKA) families and its gene has thus been termed pkb. Expression of the pkb gene is general but highest in brain, heart and lung. Translation of pkb RNA in vitro generated a 57-kDa protein (PKB) recognised by antisera raised to a bacterially expressed PKB/TrpE fusion protein. Transfection of COS cells with the kinase cDNA resulted in the synthesis of a 60-kDa protein which was partially purified by Mono Q anion-exchange chromatography. Column fractions containing PKB-immunoreactive protein exhibited elevated histone H1 kinase activity compared with similar fractions from control cells, demonstrating the enzymatic activity of this protein kinase.