From root canals of 13 teeth with necrotic pulps, samples were taken for analysis of endotoxin content by means of the Limulus lysate technique. The results were related to the viable bacteria count of root canal samples, pheniol/water-extracted lipopolysaccharides (LPS) from different gram-niegative anaerobic bacteria, and to different fractions of intflammation exudate. It was found that the endotoxic activity of the root canal samples was correlated with the presence and the number of gram-negative bacteria in the root canal.