Technical Abstract: Background: Selenium (Se) supplementation may reduce cancer risk in subjects with initial plasma Se concentrations <106 ng/ml. It would be useful to know the Se intake required to achieve such a threshold. Objective: Characterize the responses of multiple biomarkers of Se status in healthy adults during a year-long intervention with selenomethionine (SeMet). Design: A total of 261 men and women were randomized to four doses of Se (0, 50, 100 or 200 µg/day as L-SeMet) for 12 months. Responses of several biomarkers of Se status (plasma Se, plasma seleoprotein P [SEPP1], plasma glutathione peroxidase activity [GPX3], buccal cell Se, urinary Se) were determined relative to genotype of four selenoproteins (GPX1, GPX3, SEPP1, selenoprotein 15 [SEP15]), dietary Se intake and parameters of single-carbon metabolism. Results: Supplemental SeMet did not affect GPX3 activity or SEPP1 concentration, but produced significant, dose-dependent increases in the Se contents of plasma, urine and buccal cells that plateaued by 9-12 mos and were linearly related to effective Se dose (µg/day/kg0.75). The increase in urinary Se excretion was greater for women than men, and for individuals of the GPX1 679 T/T genotype than for those of the GPX1 679 C/C genotype. Conclusions: The most responsive biomarkers of Se status were those related to body Se pools: plasma, buccal cell, urinary Se concentrations. The change in plasma Se resulted from increases in its non-specific component. For non-deficient adults, the Se intake (as SeMet) required to support plasma Se concentration at a target level (Sepl-target) is: Sein= (Sepl-target – Sepl)/18.2 ng d kg0.75 ml-1ug-1.