Experiments were done to investigate mechanisms of appearance of both 7 S and 11 S IgA in nasal secretions. Three IgA preparations, (a) 11 S IgA from nasal secretions, (b) 7 S IgA from homologous serum, and (c) 7 S IgA from autologous serum, were isolated, labeled, and injected intravenously into volunteers. The rate of disappearance from plasma and the extent and nature of their appearance in nasal secretions were examined in detail. After intravenous injection, 11 S IgA from nasal secretions disappeared rapidly from plasma. However, only negligible amounts of 11 S IgA were detected in nasal secretion, which suggested that rapid selective secretion of circulating 11 S IgA does not occur as a mechanism of IgA accumulation in nasal secretions. Both the homologous and autologous 7 S IgA preparations disappeared from plasma at a normal rate, and both appeared in nasal secretions unchanged in sedimentation behavior. The specific activity of IgA in nasal secretions, when related to the total IgA concentration, was about 30-fold less than that in serum. When related to only the 7 S IgA concentration of nasal secretions, the specific activity was about one-half that of serum. These studies are consistent with the following hypotheses: (a) circulating 7 S IgA is a source of part of the 7 S IgA found in nasal secretions. The remainder of the nasal secretion 7 S IgA may be synthesized locally in the tissues of the upper respiratory tract; (b) 11 S IgA in nasal secretions is not assembled from serum 7 S IgA components; and (c) 11 S IgA in nasal secretions is synthesized de novo in the tissues of the upper respiratory tract.