Secretory component (SC) isolated from human colostrum was radioiodinated with chloramin-T method and it was subjected to a radioimmunoassay for quantitation of SC. A monospecific rabbit anti-sera against human SC which could react with free SC as well as immunoglobulin bound SC was used for the assay. The present method can measure as low as 300 ng/ml of SC. The level (mean +/- 2 S.D.) of serum SC in healthy subjects was determined to be 31.3 +/- 7.2 micrograms/ml. Studies with Sephadex G-200 gel filtration demonstrated that SC in the normal sera existed mostly as a form of SC-IgA. In contrast, free SC was never detected in them.