Endotoxin in persistent middle ear effusions (MEEs) from children with acute otitis media (AOM) whose acute inflammation had cleared was assayed by Limulus test, parallel with the detection of bacteria. Seventy-five percents of MEEs were endotoxin positive, and the concentration of endotoxin in MEE was not related to the interval between the onset of AOM and the date of sample collection. However, the cases with MEE that had high concentration of endotoxin tended to have persistent MEE for more than one month after sample collection. Therefore, the presence of endotoxin may be one of the causal factors of a delayed recovery of AOM and of the development of otitis media with effusion.