A cDNA encoding feline interleukin 15 (IL15) was cloned from the lymph node of a cat infected with feline infectious peritonitis virus. The cDNA is 486 bp in length and encodes a protein of 162 amino acids. Recombinant protein was readily expressed as a GST fusion in Escherichia coli and purified by glutathione affinity chromatography. Expression of recombinant protein in mammalian cells was only accomplished by eliminating the 5' and 3' UTR, replacing the IL15 signal peptide with the tissue plasminogen activator signal peptide, and adding 3' sequence to disrupt presumptive secondary structure of the mRNA. Biologically active feline IL15 was expressed in HEK293T cells and was shown to sustain primary feline lymphocytes, a feline T cell line, and mouse CTLL-2 cells. Proliferation of CTLL-2 cells was induced by the recombinant protein in a dose-dependent manner. Monoclonal and polyclonal antibodies against human IL15 recognized feline IL15 in immunofluorescence and Western blot assays. Additionally, feline IL15 was detectable using a commercially available human IL15 ELISA kit.