U-notched, four-point bend tests of 1090 steel were performed in the, presence or absence of precharged hydrogen. Failure occurred in a mode II manner under mixed mode I–II loading, with cracking preceded by plastic instability in the form of surface rumpling and shear localization. The critical strain for surface instability was less in compression than in tension and was reduced by hydrogen, markedly in the tension case and weakly in the compression case. The results are discussed in terms of models for plastic instability and for hydrogen degradation.