Ras/myc-transformed serum-free mouse embryo (ras/myc SFME) cells were treated with interferon-gamma (IFN-γ, 100 units/ml) and/or lipopolysaccharide (LPS, 0.5 μg/ml) for 24 h to investigate the effects of these ligands on the expression of matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase-1 (TIMP-1). Aminoguanidine (AG, 1 mM; a nitric oxide synthase [NOS] inhibitor) was also added along with IFN-γ and LPS to analyze a possible association of NO with invasiveness. Treatment of cells with IFN-γ alone did not alter MMP-9 mRNA expression or pro-MMP-9 production, but LPS alone and IFN-γ + LPS co-treatment enhanced them significantly. TIMP-1 mRNA expression remained unchanged with or without treatment and the mRNA expression of MMP-9 exceeded that of TIMP-1 in LPS- or IFN-γ + LPS-treated cells. Co-treatment of cells with IFN-γ and LPS up-regulated invasiveness and indicated a possible involvement of NO in the enhancement of invasiveness. These results suggest that ras/myc SFME cells respond to inflammatory and infectious conditions and that they may possibly modulate their characteristics as cancer cells due to their increase in MMP-9 expression and invasion activity.