The purpose of this study was to determine whether bacterial surface components other than lipopolysaccharide (LPS) could stimulate pro-inflammatory cytokine synthesis by mesenchymal and myelomonocytic cells in vitro. LPS, lipid A-associated proteins (LAP) and saline-extractable surface-associated material (SAM) were isolated from the periodontopathogenic bacterium Actinobacillus actinomycetemcomitans and added to cultures of human gingival fibroblasts (HGFs), human PBMCs and the human myelomonocytic MonoMac-6 cell line. Pro-inflammatory cytokine release into culture supernatants was determined by two-site ELISAs. Contrary to expectation, the highly purified LPS extracted from this bacterium was significantly less potent than the other surface extracts in stimulating release of IL-1 beta, IL-6 and TNF-alpha by all three cell types. The SAM was the most potent cytokine-stimulating agent showing equivalent activity to highly purified E. coli LPS in stimulating IL-6 release by PBMCs. LAP also had cytokine-stimulating activity although it was generally significantly less potent than the SAM. Thus in the case of this organism, which is involved in the pathology of chronic inflammatory diseases the LPS does not appear to be the major cytokine-stimulating component.