A novel intravenous therapy consisting of polyvalent IgG antibodies to lipopolysaccharide (LPS, endotoxin) obtained from screening of blood donors was used for treatment of patients with profound septic endotoxin shock. Investigation of the anti-LPS IgG pharmacokinetics in the 10 patients revealed time related changes in the plasma concentrations of anti-LPS IgG, endotoxin, tumour necrosis factor (TNF) and the clinical parameters. A decrease in serum concentrations of IgG and IgM antibodies to LPS was observed prior to the immunotherapy as well as in a clinical example of lethal septicemia without anti-LPS immunotherapy. Increasing serum concentrations of anti-LPS IgG during antibody infusion was followed by a decrease in the concentration of endotoxin and TNF. In survivors an IgM and IgG anti-LPS antibody response developed. Using clinical parameters and APACHE II clinical severity scores to measure the clinical condition, a beneficial effect was observed within 24 h corresponding to a decrease in the calculated expected mortality rate from more than 80% to about 50%. Five patients (55%) expired during the study. One patient died in the early septic shock phase. One patient expired due to superimposed hemorrhagic shock. Three immunosuppressed patients died 1-2 weeks after initial recovery, 1 with fungal sepsis and 2 patients due to pseudomonas infection.