A cDNA containing the porcine Toll-like receptor 4 (TLR4) coding sequence has been cloned by RT-PCR from alveolar macrophages mRNA, and its complete sequence has been determined. The predicted amino acid sequence comprises an extracellular domain with 21 leucine-rich repeats (LRR) and a LRR-C-terminal (LRR-CT) motif, followed by a 30 amino acid transmembrane segment, and a 179 amino acid intracytoplasmic region containing the Toll/IL-1R domain. Pig TLR4 shows 63-80% amino acid sequence identity with those of cow, horse, cat, human, rabbit and mouse. The degree of sequence identity rises to over 90% in the TIR domain. The whole TLR4 sequence and its ectodomain were expressed as GFP fusion proteins in CHO cells. Using RT-PCR analysis, porcine TLR4 transcripts were detected in DCs, monocytes and macrophages, and in tissue samples of bone marrow, thymus, lymph node, spleen, brain, liver, kidney and ovary. The expressed protein will be used for the development of reagents. Knowledge of TLR4 expression will help to address mechanisms of immune induction by antigens and vaccines.