Rat CD14 cDNA clones were isolated. The predicted protein sequence exhibits 82, 61.6, and 64% identity with the mouse, rabbit, and human CD14, respectively. The levels of rat CD14 mRNA expression in resident peritoneal macrophages (PM), alveolar macrophages (AM), and peripheral blood monocytes (BM) were constitutively high, whereas that in Kupffer cells (KC) was low. On intravenous injection with lipopolysaccharide (LPS), the expression of rat CD14 mRNA in KC increased markedly, whereas the increases in PM, AM, and BM were mild. Similar features of expression of rat CD14 in these cells were observed after stimulation with LPS in vitro. The level of tumor necrosis factor alpha (TNF-alpha) mRNA expression in KC after stimulation with LPS in vivo was comparable to that in PM, AM, and BM, whereas that of TNF-alpha mRNA expression in KC and PM after stimulation with LPS in vitro was lower than that in AM and BM. Interleukin (IL)-1 beta and iNOS mRNA expressions in KC after stimulation with LPS in vivo and in vitro were low, whereas those in PM, AM, and BM were high. Little or no expression of IL-6 was observed in KC after stimulation with LPS in vivo and in vitro, whereas higher expression was observed in PM, AM, and BM than in KC.