Quantification of IgD was performed by ELISA in 180 plasma samples and 83 nasopharyngeal secretions (NPS) from children aged 2-162 months with varying degrees of recurrent otitis media. Furthermore, in 24 of the children the density of the IgD-immunocytes (IgD-cells) was calculated in immunoenzyme-stained cross-sections of their nasopharyngeal tonsils (NPT). Owing to a considerable variation of the IgD cell density throughout the NPT, a semi-quantitative counting system was applied. An irregular distribution of plasma IgD was observed during childhood and maximum levels were found between 48 and 72 months of age. However, an even distribution of plasma IgD was found among the four groups of children investigated. Based on calculations of the transudation of albumin from plasma to the NPS the amount of locally produced IgD in NPS (NPS-IgD (local] was estimated to 88% (range 41-99%). Significantly higher levels of NPS-IgD (local) were found in otitis-prone children than in the other groups. Moreover, a positive correlation was calculated between levels of NPS-IgD (local) and NPT-IgD cell density, indicating that NPT, being the local lymphoepithelial tissue, also functions as an important source of NPS-IgD. NPS-IgD was not found to be associated with secretory component, indicating a passive transfer of IgD through the mucosal membranes. Our results support the hypothesis of an association between the occurrence of IgD in the mucosa and secretions of the upper respiratory tract with localized inflammatory events.