The immunoglobulin content of duodenal fluid in children at different ages has not as yet been determined, while that of serum, feces, and saliva has been well established. In 92 normal infants, children, and young adults on whom duodenal intubation was performed, duodenal fluid IgA, IgM, and IgG levels were measured. From these data, a developmental profile of immunoglobulins in duodenal fluid from 2 weeks of life to 19 years was obtained. Duodenal fluid IgA, IgM, and IgG appeared to maintain rather constant levels. The grand mean IgA level was lower than the mean IgG level (12.7 +/- 0.8 vs 29.1 +/- 2.5 mg/g of protein). Secretory component was detected in all duodenal fluids. IgG concentrations were greater than those of IgM (29.1 +/- 2.5 vs 13.2 +/- 1.4 mg/g of protein) over all age ranges. The developmental patterns of all immunoglobulins appeared to be different from those observed in serum or saliva with the exception of duodenal fluid IgA, which has a pattern similar to that of salivary IgA.