In the temperature range T = 5−80 K, the CW X-band EPR method was used to study blood and its serum samples taken from professional athletes, patients with connective tissue dysplasia and volunteers from the control group. At the measurement temperature of 5 K, a fairly intense absorption line with an effective g-factor of ≈ 5.84 was recorded in almost all EPR spectra of blood serum, the intensity of which decreases with increasing temperature. At temperatures above 40 K, this signal is not recorded. We believe that this absorption line arises from high-spin Fe3+ ions bound to the second conformation of the transferrin N-lobe iron site with E/D ≤ 0.1. Simulation of the X-band EPR spectrum of serum recorded in the range of 500−2500G at 5K using the EasySpin software package, in particular the “pepper” utility, allowed us to describe well all the features of the experimental spectrum from human transferrin: a double peak, a shoulder, wide wings of the signal with g′ ∼ 4.3; a weak line with g′ ∼ 9.0 and, most importantly, a signal with g′ ≈ 5.84.