Acute otitis media (AOM) has been epidemiologically related to viral respiratory tract infections, and viral antigens have also been detected in middle ear secretion in some AOM episodes. Successive serum samples from children followed prospectively for three years from birth were analysed for IgG antibodies against respiratory syncytial virus (RSV), adenoviruses and influenza A virus. Values from serum antibody activity gradually decreased during the first six months of life, followed by a gradual increase. Various relationships were found to obtain between age and the increases of antibody activity against the different viruses. Thus, three quarters of those tested had manifested increased antibody activity against RSV by 18 months of age, and against adenoviruses by 30 months of age. No increase of antibody activity against influenza A was noted before 12 months of age, and then only seen in two thirds of those tested during the entire three-year observation period. With regard to age, however, the proportion of children with increased antibody activity to RSV, adenoviruses or influenza A virus did not differ between otitis-prone and non-otitis-prone children. Thus, as compared to non-otitis-prone children, development of the ability to produce antibodies against these viruses was not found to be delayed in otitis-prone children.