Porphyromonas gingivalis is a Gram-negative anaerobic oral black-pigmented bacterium closely associated with chronic periodontitis. Lipopolysaccharide (LPS) derived from P. gingivalis is shown to be unusual because the LPS contains a greater number of lipid A species, such as tri-, tetra-, and/or penta-acylated lipid As. In this study, a lipid A possessing penta-fatty acyl chains derived from P. gingivalis strain 381 (compound PG-381-5FA) was synthesized, and examined for its immunobiological activities, compared with a tri-acylated lipid A (compound PG-381-3FA) synthesized previously. Compound PG-381-5FA, similar to compound PG-381-3FA, demonstrated weaker activity in a Limulus test as compared with Escherichia coli-type synthetic lipid A (compound 506). Compound PG-381-5FA, followed by compound PG-381-3FA, induced KC, interleukin-6, and tumour necrosis factor-alpha production in peritoneal macrophages from LPS-responsive C3H/HeN mice, but not in those from LPS-hyporesponsive C3H/HeJ mice. Furthermore, compound PG-381-5FA, as well as compound PG-381-3FA, activated nuclear factor-kappa B via Toll-like receptor (TLR)4/MD-2, but not TLR2, in a manner similar to compound 506, and worked as an antagonist for compound 506-induced cell activation. In the case of human peripheral blood mononuclear cells, compound PG-381-5FA showed much stronger IL-6-inducing activity than compound PG-381-3FA. The present results demonstrate that the chemical synthesis of a penta-acylated lipid A, mimicking the natural lipid A portion of LPS from P. gingivalis, is attributable to immune cell activation through TLR4, similar to that of compound 506.