We assayed mitogen-activated protein (MAP) kinase phosphorylation in a human monocyte cell line (THP1) during their infection by Toxoplasma gondii. In addition, we tested the effect of specific MAP kinase inhibitors (PD098059 and SB203580) on parasite invasion. MAP kinase phosphorylation was increased in the cytosol and membrane fractions of THP1 infected with T. gondii. The MAP kinase phosphorylation of uninfected THP1 cells was not significantly modified by incubation for 20 h with 1000 U/ml of IFN-gamma. However, IFN-gamma treatment of infected cells significantly reduces the increase in phosphorylation caused by parasite infection. There was also MAP kinase activity in the cytosol and membrane fractions of extracellular T. gondii tachyzoites. IFN-gamma altered the distribution of activity in subcellular fractions of extracellular T. gondii tachyzoites. This indicates that IFN-gamma directly affects parasite MAP kinase activity. The results provide evidence that MAP kinase pathways participate in the infection by T. gondii and that the decrease in MAP kinase activity in infected cells caused by IFN-gamma may be involved in mediating their protective signals.