Exudate was collected from germfree rats subjected to experimental intestinal strangulation obstruction. The exudate, called strangulation fluid, was non-lethal when injected into the peritoneal cavity of mice. The fluid was found to enhance the virulence of E. coli towards mice. Strangulation fluid from germfree rats appeared to contain less opsonic and bactericidal properties than serum from germfree rats towards E. coli. The fluid appeared to inactivate the opsonic and bactericidal activity of conventional rat serum to a substantial degree. Substances which interfere with host defence against bacterial infections are probably released from host tissues during intestinal strangulation obstruction.