A feature common to all members of the protein-tyrosine kinase (PTK) family is a highly conserved catalytic domain which is characteristic of this group. Degenerate oligonucleotide primers corresponding to two of the most highly conserved regions of the PTK catalytic domain were designed to amplify cDNA sequences of restricted subfamilies of PTKs from rat brain mRNA in the polymerase chain reaction (PCR). A third degenerate oligonucleotide primer corresponding to a highly conserved, PTK subfamily-specific sequence located between the two sequences mentioned above was also used to amplify cDNA sequences of PTKs of novel subfamilies from rat brain mRNA. pBluescript PCR libraries were constructed from the PCR-amplified cDNA. The PCR libraries were then screened by DNA sequencing for PTK-related sequences. Several sequences were identified that, on the basis of sequence comparison with known PTKs in GenBank, may encode new PTKs.