Mice lethally infected with the C56 strain of Toxoplasma gondii and treated with purified recombinant murine tumour necrosis factor (TNF, 1 microgram/day/mouse for 8 days), recombinant human interleukin-1 (IL-1 alpha or IL-1 beta, 100 ng/day/mouse for 5 days) or a single dose of a combination of TNF (1 microgram/mouse) and IL-1 alpha or IL-1 beta (100 ng/mouse) were significantly protected against death (P less than 0.05-0.001, as compared with untreated infected controls). Mice infected with 100,000 tachyzoites of the highly virulent RH strain of T. gondii released serum TNF in relation to the time after infection and were primed to secrete an enhanced level of serum TNF upon stimulation with bacterial lipopolysaccharide (LPS). In vitro studies showed that interferon-gamma (IFN-gamma) increased the antimicrobial activity of murine peritoneal macrophages whereas TNF, IL-1 alpha and IL-1 beta did not. TNF, however, synergized with the anti-toxoplasmic effect provided by IFN-gamma and this activity was blocked by anti-TNF antibodies. IFN-gamma induced the production of TNF and the anti-toxoplasmic effect provided by IFN-gamma seemed to be dependent partly on the production of TNF. We conclude that TNF and IL-1 may play a significant role in modulating the host's immune defence against T. gondii infection.