Fifty-eight infants and children with acute otitis media were prospectively studied for bacterial and viral pathogenesis and response to antibiotic therapy. Tympanocentesis for bacterial and viral cultures of middle ear fluids (MEF) was done before and 2-4 days after beginning treatment. Patients were followed until the end of antibiotic course. Bacteria were cultured from the preantibiotic MEF in 43 cases (74%). Viruses were cultured from the preantibiotic MEF in 11 cases (19%); all of these MEFs also contained bacterial pathogens. A significantly higher proportion of patients with both virus and bacteria (50%) failed to respond with clearing of bacteria 2-4 days into therapy compared with the group with bacteria alone (13%). The patients with persistently positive viral cultures of the MEF seemed to have purulent otitis of longer duration. Presence of virus in the MEF may interfere with bacteriologic and clinical responses to antibiotic. The mechanism of interference deserves further investigation.