Mixed oligonucleotide probes were used to screen a HeLa cDNA library for clones encoding amino acid contiguities whose conservation is characteristic of the protein-serine kinase family. Eighty thousand clones were screened, from which 19 were identified as showing strong hybridization to two distinct probes. Four clones were chosen for characterization by partial DNA sequence analysis and 3 of these were found to encode amino acid sequences typical of protein-serine kinases. One deduced amino acid sequence shares 72% identity with rabbit skeletal muscle phosphorylase kinase gamma-subunit, while another is closely related to the yeast protein-serine kinases CDC2 in Schizosaccharomyces pombe and CDC28 in Saccharomyces cerevisiae. This screening approach should have applications in the identification of clones encoding previously unknown or poorly characterized members of other protein families.