Interferon-γ has been suggested as a cytokine of connective tissue stabilizer. In addition, it has also been demonstrated that this cytokine inhibited bone remodeling activities of the bone derived cells. In order to illuminate the effects of this cytokine in orthodontic force induced bone remodeling, it was administered to primary cultured periodontal ligament cells which have been known to have some osteoblast like characteristics. Interferon-γ slightly decreased [^³H]thymidine incorporation rate without a significant change in the total cellular DNA content up to 1000 U/ml, which meant these doses were not cytotoxic to the cell. Total protein synthesis was not influenced by various concentration of interferon-γ whether it was determined by the [^³H]proline incorporation rate or by the Lowry smethod. The effect of interferon-γ on the individual protein was, however, differential, ie, it increased [^³H]proline incorporation into the noncollagenous protein marginally, while it decreased [^³H]proline incorporation into the collagen, so that it caused dose-dependent suppression of the relative collagen synthesis. On the contrary, the fibronectin synthesis determined by the ELISA was increased by 1000 U/ml of interferon-γ. The differential effects of the interferon-γ on the collagen and fibronectin synthesis exhibited not only their protein level but also the steady state mRNA level. Interferon-γ decreased steady state level of α1(I) procollagen mRNA significantly, while showing no significant changes in the fibronectin mRNA level. In addition to this, it was also found that indomethacin did not affect on the interferon-γ induced collagen decrease in this cell, which meant prostaglandins were not involved in the process of interferon-γ induced collagen decrease. So it can be concluded that the incubation of periodontal ligament cells with 1000 U/ml of interferon-γ for 24 hr showed differential effects on the type I collagen and fibronectin gene expression. The decrease in relative collagen synthesis in the protein level was related with decrease in the steady state level of mRNA, while the increase in the fibronectin synthesis in the protein level was not correlated with the mRNA level.