The effects of interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), interleukin 1-alpha (IL-1-alpha), and interleukin 6 (IL-6) on the growth of the Toxoplasma gondii RH strain were studied in vitro using a human astrocytoma-derived cell line. Cells were treated with cytokines at different concentrations at 24 h prior to infection with T. gondii tachyzoites. IFN-gamma did not induce any modification in T. gondii growth, whatever the concentration used. TNF-alpha induced a significant decrease in the total number of tachyzoites, whereas IL-1-alpha surprisingly induced an increase in the number of tachyzoites. Our results show that the effects of cytokines on T. gondii growth may be of great importance in the control of cerebral toxoplasmosis but that they can vary, depending on the cell type considered.