In connection with grommet insertion due to secretory otitis media (SOM), the middle ear effusion was collected and studied for content of interferon and bacteria. The study material comprised a total of 47 ears in 29 children, but from 10 ears the effusion was too viscous for interferon titration, so only 24 children were ultimately included. Interferon was present in 5 ears of 3 children. A child having a low titre in both ears had recently been treated with penicillin. From the other 2 children, pneumococci were cultured. Twelve of the included children had a common cold, but of them only 2 had interferon in the middle ear. The production of interferon was presumably induced by the pneumococci, not by virus.