Ab initio quantum calculations are applied to both the parallel and the antiparallel arrangements of the beta-sheets of proteins. The energies of the NH center dot center dot center dot O and CH center dot center dot center dot O hydrogen bonds present in the beta-sheet are evaluated separately from one another by appropriate modifications of the model systems. The bond energies of these two sorts of hydrogen bonds are found to be very nearly equal in the parallel beta-sheet. The NH center dot center dot center dot O bonds are stronger than CH,,, O in the antiparallel geometry but only by a relatively small margin. Moreover, the former NH center dot center dot center dot O bonds are weakened when placed next to one another, as occurs in the antiparallel beta-sheet. As a result, there is little energetic distinction between the NH center dot center dot center dot O and CH center dot center dot center dot O bonds in the full antiparallel beta-sheet, just as in the parallel structure.