The aim of this study was to establish soluble serum transferrin receptor (sTfR) reference limits. sTfR was measured in 885 healthy subjects from 3 to 91 years old (433 men, 409 women), without hematological abnormalities, using an immunonephelometric assay. The sTfR median concentrations in our population decreased gradually from the group aged 3-10 years to the group aged 21-40 years, then there were no changes in the older groups except for the females >60 years of age. The interindividual variability ranged from 12.6% to 30.3% among different age groups, and the analytical variability was 5%. Biological factors and other factors associated with sTfR concentration variation were examined and accounted for 35% of the sTfR variability in men aged 20 years or less, and 18% in those older than 20 years. Also, they accounted for 45% of the variability in women aged 20 years or less and 14% in those older than 20 years. The main factors statistically associated with sTfR concentration in males were ferritin, orosomucoid, hemoglobin, and tobacco in all age groups and only mean corpuscular volume (MCV) in males less than 20 years old. In the females the main factors were age, orosomucoid, and hemoglobin in all age groups, MCV and tobacco in females less than 20 years old, and ferritin and physical activity in females more than 20 years old. These factors were used to define the exclusion and partition criteria for obtaining the reference samples. Medians for reference values were: 1.60 mg/l in the 3-10-year old group (males and females); 1.42 mg/l in males between 11 and 20 years of age, and 1.33 mg/l in females of the same age. In the other age groups, the median of the reference values was 1.16 mg/l, except in females over 60 years old, for whom it was 1.26 mg/l.