The transmission of 0.511-MeV photons through concrete, lead, or iron is determined using MCNP4c2, by exposing 50-cm-radius cylinders of the materials to plane parallel sources. Cylinders are modeled with thicknesses up to 50 cm in 5-cm increments for concrete, 10 cm in 1-cm increments for lead, and 20 cm in 2-cm increments for iron. The resulting transmission factors span from 1 to <10-3 for concrete, to almost 10-7 for lead, and to roughly 10-5 for iron. The reliability of the method is checked by performing the calculations for selected thicknesses of material with a 0.662-MeV source and comparing the results against published data. Acceptable agreement is reported in almost all cases.