Porins isolated from Salmonella typhimurium and found to contain less than 0.1% w/w of LPS, were found to be lethal at a dose of 100 ng to both LPS-responder (BALB/cByJ) and non-responder (C3H/HeJ) mice sensitized with D-galactosamine. This lethal action could be prevented by anti-TNF-alpha serum given intravenously 10 min before the porin injection but not by polymyxin-B mixed with the porins in a ratio of approximately 300 moles polymyxin-B per mole of porin. The porin preparation was also pyrogenic to rabbits at a dose of 1 microgram/kg and elicited a local Shwartzman reaction when used as the sensitizing and eliciting agent; these reactions were also present when the porins were mixed with polymyxin-B.