The cDNAs encoding canine and feline flt3 ligand (FL) have been isolated from libraries constructed from canine or feline peripheral blood mononuclear cells and sequenced. A full-length canine FL cDNA encodes a polypeptide of 294 amino acids that shares 52.8% and 71.0% identity with the mouse and human FL protein, respectively. While the feline FL cDNA encodes a polypeptide of 291 amino acids that shares 54.1% and 70.2% identity with the mouse and human FL protein, respectively. The canine and feline FL proteins share 82.8% identity at amino acid level. Besides those encoding the full-length FL protein, several alternatively spliced forms of canine FL transcripts were also isolated. The canine and feline FL proteins have longer cytoplasmic domain than their mouse and human homologue.