The buildup factor for gamma ray exposure is most useful in calculations for biological protective shields. The buildup factors for gamma ray exposure were calculated in two layered shields consist of water - lead and lead - water up to optical thickness 20 mean free path ( mfp ) at gamma ray energies 1, 2 and 6 MeV by using kalos's formula. The program has been designed to work at any atomic number of the attenuating medium, photon energy, slab thickness and the arrangement of materials. The results obtained in this search leading to the buildup factor for gamma ray exposure at energies ( 1 and 2 MeV ) in lead-water were higher than the reverse case, while at energy 6 MeV the effect was opposite. The calculated data were parameterized by an empirical formula as a function of optical thickness of two materials. The results obtained were in reasonable agreement with a previous work.