Jejunal fluid was collected from 40 children aged 1-12.2 years and from 10 adults (age 20-25 years) who were without gastrointestinal disorders. They were intubated pernasally with a special tube system that made it possible to avoid contamination from the upper respiratory tract secretions. Quantitation of immunoglobulin A (IgA), secretory IgA (SIgA), IgM, SIgM, IgD, IgG, and albumin was performed by double sandwich enzyme-linked immunosorbent assay. IgA was the dominating immunoglobulin in jejunal fluid, showing the highest concentrations, followed in order by IgM, IgG, and IgD. The IgD concentrations were about 1,000 times less than in plasma of age-matched children. The immunoglobulin concentrations did not show any age-related differences, except for IgG and for the SIgA/IgA ratio. The former declined with children's age (p less than 0.02), whereas the latter was significantly higher in adults (interquartile range: 97-100%) than in young children aged 1-less than 5.5 years (interquartile range: 55-94%) (p less than 0.001). This indicates an age-related decline in leaking from gut mucosa. The concentrations of SIgA and SIgM showed a positive correlation (p less than 0.001). The majority of IgA and IgM were estimated to be locally produced, whereas IgD and IgG were transudated from plasma.