The isolation and purification of lipopolysaccharides from several related strains of eukaryotic algae is disclosed. These lipopolysaccharides have been shown to be structurally and functionally similar to the lipopolysaccharides of gram negative bacteria, or endotoxins, which have been shown to be a cause of sepsis or septic shock. The present invention also relates to the use of these algal lipopolysaccharides to inhibit the release of TNF-a from macrophages, which has been associated with the initiation and escalation of sepsis as a result of bacterial endotoxins. The present invention further relates to the use of these algal lipopolysaccharides to inhibit the innate immune system of insects and mammals.