Objective: This study examined the effects of methyl mercaptan, a produet of the bacterial putrefaction of protein in periodontal pockets, on the function of cells in culture. Method and materials: Human gingival fibroblasts and periodontal ligament cells were exposed to a constant, continuous flow of methyl mercaptan in vitro. Control and test cultures were then examined for changes in intracellular pH, an event often associated with alterations in cellular function. Intracellular pH was determined by single-cell Image analysis of cells loaded with a fluorescent, pH-sensitive dye. Periodontal ligament cells were also tested for changes in synthesis of total protein and fibronectin. Results: Test cells exhibited a consistent decrease in intracellular pH following exposure to methyl mercaptan. Measurements of total protein production showed that test periodontal ligament cell cultures produced approximately 30% less protein than control cultures (P 0.05). Western-blot analysis of fibronectin in medium demonstrated that abnormal monomeric fibronectins were a major protein in test, but not in control, cell cultures. Conclusion: Exposure to methyl mercaptan induced alterations in intracellular events that paralleled changes in extracellular matrix proteins. The observed changes in extracellular matrix proteins support the hypothesis that methyl mercaptan contributes to the progression of periodontal disease. Keywords: fibronectin synthesis, gingival fibroblasts, intracellular pH, methyl mercaptan, periodontal ligament cells, protein synthesis, volatile sulfur compounds