We describe an enzyme immunoassay for human serum ferritin in which antibody adsorbed on polystyrene tubes is used. Adsorbed gamma-globulins against human ferritin were first allowed to react with ferritin and a second antiferritin antibody, labeled with alkaline phosphatase, was added. The amount of bound enzyme/antibody conjugate was proportional to the ferritin titer in the assay. This method offers stable reagents that can be kept for many months at 4 degrees C. The average values for ferritin in normal men and women were, respectively, 58 and 43 microgram/liter. The lowest detectable concentration was 5 microgram/liter.