The bovine interleukin-15 (IL-15) sequence was cloned from abomasal lymph node mRNA by enzymatic amplification of cDNA using human primers proximal to and including the translation start and stop sites. The open reading frame is 486 base pairs in length, and the proposed protein sequence shows 78.4% and 73.5% similarity with that predicted for the human and mouse sequences, respectively. Expressed and purified recombinant bovine IL-15 in the absence of the 48-amino acid leader sequence stimulated the proliferation of bovine lymphoblast cells at least 12-fold over background at maximum concentration levels. Competitive reverse transcriptase-polymerase chain reaction analysis showed constitutive levels of IL-15 mRNA within a broad range of tissues and cell types. Lipopolysaccharide addition to adherent lymph node populations caused moderate increases in IL-15 transcription, whereas the addition of phorbol 12-myristate 13-acetate and calcium ionophore failed to induce gene expression for this cytokine. Transcription of IL-15 was also downregulated in the presence of low concentrations of human recombinant interleukin-2.