Endotoxin administered together with homologous heart tissue homogenate in rats had a marked adjuvant effect in production of circulating anti-heart antibodies. This effect was greater than that of Freund's complete adjuvant and was abolished when the endotoxin was given separately, before or after the homogenate. Focal infiltrative myocarditis was seen in some of the animals but could be attributed equally to the endotoxin as to the administered antigen. It is suggested that the autoimmune process can be partially explained on the basis of modification of tissue antigen by linkage to endotoxin.