A new direct method for determination of chemical potentials from molecular dynamics simulations is proposed. This paper focuses on development and testing of the method for pure, Lennard-Jones fluids. The proposed method uses a semipermeable membrane to mimic an osmotic experiment. With only a few particles designated as those to which the membrane is permeable, the chemical potential relative to that of an ideal gas can be determined for a fluid at virtually any realistic temperature and density, and only the equilibrium mole fraction of the permeable particles in the "mixture" side of the cell is required. The chemical potentials obtained using the new osmotic method agree well with those obtained from the Lennard-Jones equation of state and those obtained from Widom’s method. The new method has the main advantage over traditional methods of being extendable to complex molecules because no particle insertions are required.