The ambiguous relationship between fatigue crack growth rate and hydrogen concentration C-H in the bulk of metal under cyclic loading of the ferrite-pearlite low-alloyed steel in hydrogen-contained environments has been found: there is a certain C-H value at which the crack growth resistance of steel increases. At these test conditions fracture surface demonstrates some increasing of the plastic component on relief. The received results point on key importance of the determining of such threshold hydrogen concentration values, which correspond the transition "enhanced plasticity - embrittlement". (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.