The small-dose iron tolerance test (SD-ITT) was performed in 37 healthy subjects (20 females and 17 males). The area under the curve (AUC) of serum iron variations after the test dose (10 mg of iron as iron sulphate) was corrected by the expected plasma iron disappearance rate, the expected subject's plasma volume and the measured spontaneous time-dependent serum iron variations, and was used as a summary measure of the outcome, Q-ITT. Q-ITT correlated strictly with the maximum serum iron increase (SImax). Q-ITT gave positive (greater than zero) values in only 14 out of the 37 subjects (11 females and 3 males). Serum ferritin proved to be the best discriminating parameter between positive and non-positive subjects, and was inversely correlated with Q-ITT in the positive ones. In 2 male subjects, aged 43 and 34 years, SD-ITT proved to be highly sensitive to the progressive decrease of mobilizable body iron content during repeated venesections. In these patients the threshold for a positive test result was obtained at values lower than 1050 and 950 mg of body iron content, respectively. The threshold-dependent sensitivity, simplicity, and repeatability of this method favor its becoming a useful technique for studying the up-regulation of iron absorption in normal subjects and in pathological conditions.