Poly(S-benzyl-L-cysteine) (PBLC) is a S-benzyl derivative of the natural amino acid poly(L-cysteine). Its normal modes and their dispersion in antiparallel beta-sheet conformation have been obtained within the first Brillouin zone using Wilson's G.F. matrix method, as modified by Higgs for an infinite polymeric chain. The zone center frequencies and the Urey Bradley force field have been obtained by "best fitting" the observed Fourier transform IR (FTIR) absorption bands. The characteristic features of dispersion curves such as crossing, repulsion, and exchange of character are discussed. Predictive values of heat capacity in the temperature range of 50-500 K are reported.