Myelin basic protein, isolated from central nervous system tissue and an inducer of experimental allergic encephalomyelitis in animals, has been demonstrated to form a stable molecular complex with the lipid A region of gram-negative bacterial lipopolysaccharides (endotoxins). This binding of endotoxin with myelin basic protein results in generation of lower m.w. aggregates with decreased isopycnic density. A number of lipid A-induced characteristic properties of endotoxin, such as B lymphocyte proliferative response in C3H/St mice, complement activation of normal human serum, Limulus lysate gelation, and lethal effects in mice, are modified as a result of binding of myelin basic protein with lipopolysaccharides.