Susceptibility to infections is a well known feature of Down's syndrome. The possible relation between this predisposition and the serum concentrations of the IgG subclasses was studied in 38 children with Down's syndrome aged 1-12 years. An age matched group of 50 healthy children served as controls. The serum concentrations of IgG1 and IgG3 were significantly raised among children with Down's syndrome in all three age groups studied (that is 1-2.5, 4-8, and 9-12 years). The serum concentrations of IgG2 were normal in the first two groups but significantly reduced in the third age group. In contrast, the concentrations of IgG4 among children with Down's syndrome were significantly reduced in all three age groups. Moreover, among the children with Down's syndrome aged 4-12 years 68% (15/22) had IgG4 concentrations below 2 SDs of the geometrical mean of the controls. The results may partially explain the proneness of children with Down's syndrome to infections with encapsulated bacteria. Although the underlying cause of these abnormalities is unknown, IgG subclass determination seems relevant in the clinical evaluation of children with Down's syndrome.