High titres of endotoxin as measured by the Limulus test were usually found in burned patients who had raised body temperatures, and were colonized with gram-negative bacteria; also some infected patients showed raised endotoxin without a raised temperature. Patients vaccinated with an antipseudomonas vaccine rarely showed endotoxin in their plasma but occasional plasma samples from vaccinated patients had a high titre of endotoxin which appeared unrelated to infection or to a raised temperature in the patient.