The CD154 (CD40 ligand) molecule is a member of the tumor necrosis factor (TNF) family and plays an important role in the interaction between antigen-specific lymphocytes and antigen-presenting cells. In this study, reverse transcription-PCR cloning was used to derive the sequence encoding ovine CD154. Sequence analysis of the cloned CD154 gene showed a similarity of 97, 89 and 88% with the bovine, porcine and human sequences, respectively, at the nucleic acid level. The deduced amino acid sequence for the ovine CD154