An analytical study has been completed to define minimum critical mass parameters for a spherical region containing a 235U-H2O mixture reflected by a natural uranium-water mixture. The study indicates that for an optimum natural-H2O reflector, the calculated minimum critical mass for the central sphere lies in the range of 75 to 200 g of 235U. The analytic study was divided into three phases. In the first phase homogeneous two-region systems were examined using diffusion theory programs. The results indicated a minimum critical mass of 451-g 285U for an optimal system. In the second phase a Monte Carlo program was utilized to compute keff for uranium metal rod-H2O moderated lattices for which experimental data are available. The purpose of the second phase was to establish the validity of the Monte Carlo program as an analytical tool. In the third phase the Monte Carlo program was used to calculate 235U + H2O spheres closely reflected by an optimum lattice of natural uranium rods. The Monte Carlo program indicates that the minimum critical mass in an H2O-moderated system reflected by a uranium metal lattice is 135 ± 65 g 235U.