Analyses of effusions and sera from patients with otitis media with effusion demonstrated local production in the middle ear of lysozyme, IgA and IgG. The effusion IgM was markedly elevated in some patients, also indicating local production. Complement C3 with rare exception was significantly lower in effusions than sera, suggesting utilization of complement in the middle ear, perhaps in conjunction with antibodies. The presence of high levels of lysozyme and immunoglobulins in effusions correlates with the low isolation rate of microorganisms in culture and may influence survival of organisms in the middle ear.