Passive immunotherapy with anti-lipopolysaccharide hyperimmune equine plasma (Anti-LPS) is effective in treating experimental Gram-negative bacterial infections. The bactericidal activity of anti-LPS towards five different Pseudomonas species, including two multiresistant Pseudomonas aeruginosa isolates was tested here, as well as the ability of anti-LPS to inhibit the quantitative chromogenic limulus amoebocyte lysate (LAL) assay. Anti-LPS caused a mean reduction of 84.4 +/- 3.2% (P less than 0.001) in the number of colony forming units (cfu) of all isolates, whereas saline and complement inactivated anti-LPS induced no reduction. Nonimmune control plasma caused a small reduction in % cfu but much less than anti-LPS hyperimmune plasma (13.5% vs 84.4%, P less than 0.001). In order to cause 99% inhibition of the LAL test of 5 ng/ml Pseudomonas aeruginosa LPS, IgG antibodies were required in 10(5)-fold excess. These results suggest that anti-LPS has potential in the therapy and prophylaxis of Gram-negative bacterial infections, especially where LPS is involved in the disease process.