Development criterea for sulfur (S), sufur plus gold (S+Au), and reduction (R) sensitized emulsions were studied through the analysis of experimental characteristic curves obtained with mono-grainlayers of the emulsions on the basis of the Poisson statistics. Under a certain range of developing time-though quite narrow for S sensitized emulsion-both the position against log exposure axis and the shape of each characteristic curve did not change and the latter fitted a theoretical curve of a single threshold number of effective photons to make a grain developable, i. e., 4, 3, and 2 for S, S+Au, and R sensitized emulsion, respectively. Gold latensification had no effects on S + Au sensitized emulison while affecting S sensitized one to give the curve for a two-effective quanta threshold. These results were discussed in the light of the current theories of latent image formation in chemically sensitized photographic emulsions.