At the slow strain rate tensile tests done using the specially designed facility, the decrease in the elongation to fracture, reduction of area, fracture energy and no effect on the strength have been stated for the low alloy ferrite-pearlite and sorbite steels, polarized in synthetic sea water at potentials corresponding to the cathodic protection (À 800 to À 1400 mV SCE ). Presence of SRB promotes the plasticity loss, being especially pronounced at potentials À 1100 to À 1200 mV SCE . At higher cathodic polarization, the plasticity estimated in inoculated and in sterile water equalizes. The effects have been correlated with the contents of absorbed and of permeable hydrogen. The promotion of hydrogen charging and the plasticity loss by SRB at the low and medium applied cathodic polarization has been accounted for the observed production of S À2 ions and inhibition of deposit formation. The negligible effect of SRB at the high cathodic polarization has been suggested to be a result of the suppression the SRB growth due to the high alkalization of the near surface solution. The same amount of hydrogen produces the less detrimental effect on the sorbite than on the ferrite-pearlite steel. However, at the similar cathodic polarization, the sorbite steel absorbs the highest amount of hydrogen and reveals the most pronounced degradation. Cathodic protection of constructions subjected to the action of SRB in the sea water should provide the conditions, under which no fragment of marine construction could be polarized by potential corresponding to the maximum degradation of the plastic properties of steels (À 1100 to À 1200 mV SCE ).