A sensitive solid phase radioimmunoassay method was established for the specific quantitative determination of secretory IgA (sIgA) by taking advantage of the dual antigenicities of sIgA, one specific for α‐chain and the other for secretory component (SC). The sIgA and IgA in the sample were first bound by anti‐IgA antibodies coated on the polystyrene tube, then the amount of bound sIgA was quantified by the use of 125I‐labeled anti‐SC antibodies. This method is quite sensitive and allows us to distinguish sIgA from IgA and free SC which usually coexist in exocrine secretions. Linear relationship was observed between the bound radioactivity of radioiodinated anti‐SC and the amount of sIgA in the range of 5 to 60 ng of sIgA. With this method, the urinary sIgA levels in normal children and adults were measured. Urinary sIgA was detected in half of infants within 7 days after birth, while it could be demonstrated in almost all of the infants by 14th day. Then, it gradually increased and reached about a half of the adult level by the age of puberty. This method will be useful for the sensitive and specific measurement of sIgA in various exocrine secretions.

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Titers are rearranged for 150mg protein concentration per ml.