The activities to induce TNF-α production by a monocytic cell line, THP-1, and ICAM-1 expression and IL-6 production by human gingival fibroblasts were detected in plural membrane lipoproteins of Mycoplasma salivarium. Although SDS-PAGE of the lipoproteins digested by proteinase K did not reveal any protein bands with molecular masses higher than approximately10 kDa, these activities were detected in the front of the gel. A lipoprotein with a molecular mass of 44 kDa (Lp44) was purified. Proteinase K did not affect the ICAM-1 expression-inducing activity of Lp44, but lipoprotein lipase abrogated the activity. These results suggested that the proteinase K-resistant and low molecular mass entity, possibly the N-terminal lipid moiety, played a key role in the expression of the activity. The N-terminal lipid moiety of Lp44 was purified from Lp44 digested with proteinase K by HPLC. Judging from the structure of microbial lipopeptides as well as the amino acid sequence and infrared spectrum of Lp44, the structure of the N-terminal lipid moiety of Lp44 was speculated to be S-(2, 3-bisacyloxypropyl)-cysteine-GDPKHPKSFTEWV-. Its analogue, S-(2, 3-bispalmitoyloxypropyl)-cysteine-GDPKHPKSF, was synthesized. The lipopeptide was similar to the N-terminal lipid moiety of Lp44 in the infrared spectrum and the ICAM-1 expression-inducing activity. Thus, this study suggested that the active entity of Lp44 was its N-terminal lipopeptide moiety, the structure of which was very similar to S-(2, 3-bispalmitoyloxypropyl)-cysteine-GDPKHPKSF.