The activation of complement and the release of TNF-α, IL-6 and IL-8 are important pathogenic factors behind organ dysfunction in sepsis. The aim of this study was to determine whether infusion of anti-TNF antibodies alters complement activation and plasma concentrations of pro-inflammatory cytokines at high doses of Escherichia coli. Six baboons received intravenously 2 × 109 live. E. coli bacteria per kg body weight (group 1), in addition five received pretreatment with 1 mg per kg body weight anti-TNF antibodies (group 2), and seven received 5 × 108 live E. coli bacteria per kg body weight (group 3). Two hours after the start of infusion of the bacteria, plasma concentrations of C3 activation products, C5a and the terminal SC5b-9 complement complex were increased in groups 1 and 2 (P < 0.05), but there was no significant difference between the groups. At 2 h the levels of TNF-α, IL-6 and IL-8 were lower in group 2 compared with group 1 (P < 0.05). In group 2 compared with group 1 the TNF-α concentrations were, however, higher at 4, 8 and 24 h. The explanation for this phenomenon is probably that TNF-α binds to the anti-TNF antibody complex and is released slowly after it has been bound. The study showed that infusion of anti-TNF antibodies reduced the concentrations of TNF-α, IL-6 and IL-8, without any detectable influence on complement activation.