The bacteriology and cytology of middle ear effusion from 729 children with persistent otitis media with effusion were studied. Thirty-five percent of these chronic effusions were culture-positive. Type b and non-type b Haemophilus influenzae, Streptococcus pneumoniae, Neisseria sp. and Staphylococcus epidermidis were the predominant isolates. Serous and mucoid effusion cultures yielded bacteria more often in younger than in older children. In addition bacteria were seen in 17% of the Gram-stained smears of the sterile effusions; Gram-positive cocci predominated in these effusions. Disparate effusion culture results were obtained in 32% of bilateral otitis media cases. Effusions which yielded H. influenzae and S. pneumoniae on culture had more polymorphonuclear leukocytes than did effusions which yielded S. epidermidis or Neisseria or were sterile. Phagocytic cells were equally prevalent in sterile effusions with or without bacteria on Gram stain. Phagocytic cells were seen less often in mucoid effusions from antibiotic-treated patients than in mucoid effusions from untreated patients. The results suggest that certain bacteria in chronic middle ear effusion contribute to the pathogenesis of this condition by eliciting a local inflammatory cell response.