A full-length cDNA clone encoding a novel trout CC chemokine was identified in expressed sequence tags generated from lipopolysaccharide (LPS)-stimulated in vitro differentiated macrophages isolated from the head kidney of the rainbow trout (Oncorhynchus mykiss). The putative 101-amino-acid protein is 38% similar to Macaca mulatta CCL4 (macrophage inflammatory protein 1beta) but is also similar to several other related mammalian CC chemokines, including human Act-2. Real-time PCR and conventional RT-PCR revealed significant up-regulation of transcript levels of the trout CCL4-like mRNA in LPS-stimulated in vitro differentiated macrophages. In unstimulated trout, CCL4-like mRNA expression was detected at different levels in all tissues tested, whereas in LPS-challenged animals (6 mg/kg), CCL4-like mRNA increased in intestine, ovary and spleen at both 24 h and 72 h post-injection. In gills, CCL4-like mRNA expression was inhibited after LPS administration. Based on the highly regulated expression pattern exhibited by the trout CCL4-like mRNA, it is likely that this chemokine plays an important regulatory role in the immune response of trout.