The response function of NaI(Tl) scintillation detectors for gamma radiation has been calculated by the Monte Carlo method. Summary results of this calculation are presented in terms of the photofraction, i.e., the ratio of the area under the photopeak to the total area of a scintillation spectrum. Values of the photofraction are given for collimated monoenergetic radiation with energies between 0.279 and 4.45 Mev incident centrally on the endface of cylindrical crystals ranging in size from 0.25 in. (radius)×0.5 in. (length) to 2.5×9 in.