The principal components of the C-13 Shielding tensors in two antibiotics, ampicillin and penicillin-V, are calculated using the coupled Hartree-Fock gauge-including atomic orbital (CHF-GIAO) method as well as using a hybrid density functional scheme. Calculated results are compared with solid state experimental nuclear magnetic resonance data. Using the known X-ray structures of these antibiotics, it is demonstrated that the computed shieldings compare favorably with experiment such that, in some cases, calculations can now be utilized in assigning shielding tensor data.