By using the polymerase chain reaction with degenerate oligonucleotides based on highly conserved motifs held in common between all members of the protein tyrosine kinase (PTK) family, a PTK-related sequence was isolated from murine peritoneal macrophage cDNA. Full-length clones have been isolated that encompass the entire coding region of the mRNA, and the predicted amino acid sequence indicates that the protein encoded has the structure of a growth factor receptor PTK (RTK). We have dubbed this molecule RYK (for related to tyrosine kinase). The RYK-encoded protein bears a transmembrane domain, with a relatively small (183 amino acid) extracellular domain, containing five potential N-linked glycosylation sites. The intracellular domain of RYK is unique among the broader family of RTKs and has several unusual sequence idiosyncrasies in some of the most highly conserved elements of the PTK domain. These sequence differences call into question the potential catalytic activity of the RYK protein.