The photonuclear activation cross section of elements whose (γ, n) cross sections lead to a suitable positron activity has been measured using monochromatic gamma rays from the T3(p, γ)He4 reaction. The gamma rays were monitored by a 3-in. diam by 4-in. long sodium iodide crystal and calibrated with a 4 1/2 ×6 in. crystal whose response curve to the γ rays was determined. The absorption coefficient of these photons in NaI was determined by a good geometry transmission experiment. The positron activity was determined by a coincidence detector, consisting of two 5-in. diam by 2-in. long NaI crystals set on the annihilation radiation photopeaks. This detector was calibrated against a F18 positron source standardized in a 2π flow counter. The C12(γ, n)C11 and F19(γ, n)F18 reactions were investigated over the range from 20.1 to 21.2 Mev. Structure, although reported by other experimenters, was not observed. The (γ, n) activation cross section was measured at 20.5 Mev for O16, Cr50, Fe54, Cu63, Zn64, Mo92, Sb121, and Pr141, giving cross section of 0.60+/-0.12, 29.1+/-6.0, 30.0+/-4.8, 52.5+/-2.5, 35.7+/-1.8, 35.4+/-2.5, 33.4+/-2.7, 51.7+/-5.5, respectively.