The effects of 6-h marathon ultra-race (long aerobic work below the lactate threshold level) on the levels of IL-6, leukemia inhibiting factor (LIF), and stem cell growth factor (SCF) were studied. The athletes participating in the study had different endurance levels evaluated by the distance covered over 6 h. The level of IL-6 sharply increased after exercise. The degree of IL-6 increase correlated with the length of the distance covered (r=0.83, p=0.042). The concentration of LIF after exercise inversely correlated with the distance covered (r=-0.75), but this correlation was statistically insignificant. The IL-6/LIF proportion exhibited the highest correlation with the result in the marathon ultra-race. This parameter most fully characterized athlete endurance (r=0.92, p=0.009). Hence, the relationship of LIF with physical endurance was demonstrated. Involvement of LIF in antibody production processes can be responsible for it.