We have identified cDNA clones that encode five new avian receptor-like tyrosine kinases of the Eph subclass, by screening two chicken embryonic cDNA libraries with DNA probes. We have designated them Cek6 to Cek10. The identification of these kinases indicates that the Eph subclass comprises at least 10 members and, therefore, represents a very large family of receptor-like tyrosine kinases. Variants of Cek10 and of Cek5 (a previously identified Eph-related kinase) containing amino acid insertion sequences in the juxtamembrane domain were also isolated. The Cek5 variant is expressed in the brain, but not in other tissues of the 10-day chick embryo. Analysis of 10-day chick embryo mRNAs shows the newly identified tyrosine kinases to be all expressed in both the embryonic brain and body tissues. In adult tissues, they display distinct patterns of expression. Cek6, Cek7, Cek8, Cek9 and Cek10 are likely to play significant roles in embryonic signal transduction pathways, including those involved in neural development. Their distinct tissue distributions in the adult suggest that the different members of the Eph family may each serve specific functions.