Eighteen periodontally diseased teeth and eleven normal teeth requiring surgical removal were each treated with one of four different solutions to elute potentially toxic material from their root surfaces. The solutions used were pyrogen‐free water, phenol‐water, trichloroacetic acid and citric acid. To define the composition of the eluted material, samples were analyzed for calcium, for ketodeoxyoctonic acid (KDO), a carbohydrate unique to Gram‐negative bacteria, and for limulus lysate activity. Limulus‐positive material was then analyzed for protein and nucleic acids and subjected to heat treatment, to the enzymes RNAse A and T1, and to lysozyme.It was found that TCA and citric acid removed more calcium and more toxic material from the root surface and subsurface than did either water or phenol‐water.The material extracted from 13 of the 18 diseased roots was limulus‐positive. Four of the limulus‐positive samples were also KDO‐positive, strongly indicating the presence of endotoxin in these samples. Neither these nor any of the other limulus‐positive responses were reversed or significantly reduced by exposure of samples to heat, to RNAse A and T1, or to lysozyme. In addition, limulus‐positive responses were unrelated to the amounts of protein and nucleic acids in the extracted material.When considered together, these preliminary characterizations indicate the presence of endotoxin in the material eluted from periodont