Bacteria were isolated from a high percentage of the effusions from patients with otitis media with effusion (OME, serous otitis media). In an attempt to determine if the isolated bacteria were involved in the disease process, we analyzed the serum and effusion of 25 OME patients for the presence of antibacterial antibodies by the indirect immunofluorescence antibody method. Specific antibody activity was detected in 20 of 25 effusions (80%) and 19 of 22 sera (86%). IgG antibodies were the most frequently found class of antibodies in both sera and effusions, but IgA antibodies were detected more frequently in the effusions than in the sera. Hemophilus influenzae, Streptococcus pneumoniae, and diphtheroids were the most frequently isolated organisms, and antibody activity to all bacterial species isolated was detected. The results support the concept that the isolated bacteria are not contaminants but are actively involved in the disease process.