Porphyromonas gingivalis and Campylobacter rectus are two major bacterial species implicated in the pathogenesis of periodontitis. P. gingivalis can antagonise the inflammatory response to other periodontal pathogens, a property commonly attributed to its lipopolysaccharide (LPS). The aim of this study was to investigate the capacity of P. gingivalis to antagonise C. rectus induced cytokine stimulation from human monocytes, and to investigate the involvement of its LPS. Primary human monocytes and Monomac-6 cells were challenged with culture supernatants from P. gingivalis and C. rectus, and levels of IL-1beta, IL-6 and IL-8 produced were measured by ELISA after 6h incubation. Purified P. gingivalis LPS was also added alone or in combination with C. rectus culture supernatant. Both species significantly stimulated the production of all three cytokines from the two cell lines, but P. gingivalis was considerably weaker inducer. Co-stimulation of the cells with P. gingivalis and C. rectus suppressed the cytokine-stimulatory capacity of the latter. P. gingivalis LPS alone was sufficient to antagonise IL-6 and IL-8, but not IL-1beta stimulation by C. rectus. In conclusion, mixed infections may impair host immune responses by reducing pro-inflammatory cytokine levels, which may be of relevance to the pathogenesis of periodontitis.