To identify novel protein tyrosine kinase (PTK) genes expressed in human lymphoid cells, we have screened B- and T-cell cDNA libraries at low stringency using a c-fms tyrosine kinase domain probe. Three new PTK genes were identified, based on the presence of conserved amino acid sequence motifs characteristic of the catalytic domain of tyrosine kinases. Of these three genes, one (tyk1) appears to be the human homologue of a previously cloned murine gene (ltk), which has been reported to encode a tyrosine kinase with a unique structure; while the second gene, tyk2 cannot be clearly assigned to any of the known PTK subfamilies, and therefore may be the prototype of a new PTK gene subfamily. The third gene (tyk3/fer) has been very recently cloned by others; we present additional characterization in this report. We have performed Northern blots to establish the size of the mRNA encoded for by these genes, and to confirm their expression in lymphoid cells. Finally, we have determined the chromosomal location of all three genes by analyzing human-mouse somatic cell hybrids.