Colon carcinoma is one of the most frequent causes of cancer death in industrialized countries. The patients generally die of the metastases. In a colon cancer rat model, the authors have shown that lipopolysaccharides from Escherichia coli induced the regression of carcinomatosis and cured 20%-30% of the rats. Some synthetic derivatives of lipid A, which are less toxic than lipopolysaccharides, were injected 14 days after the tumor cells. They induced the complete regression of peritoneal carcinomatosis consisting of numerous nodules measuring 1-5 mm in 20%-30% of rats. Only compounds with three or more hydroxymyristic acid residues were effective. In vivo effects were correlated with the capacity to induce the production of interleukin 1 and tumor necrosis factor but not with the capacity to induce macrophage-mediated cytolysis. It is therefore possible to synthesize weakly toxic derivatives of lipopolysaccharides retaining their antitumoral property in vivo.