Many physiological processes involving cell membranes are thought to involve the lateral diffusion of mobile species within the membrane. The diffusion of these mobile species is hindered by the presence of immobile species, and it would be useful to be able to evaluate quantitatively the effect of obstacles on diffusion rates. The proposed research uses Monte Carlo calculations of diffusion on a lattice to examine several questions related to lateral diffusion of tracers in the presence of obstacles. What happens in a mixture of mobile and immobile obstacles? How does the diffusion constant of a tracer depend on the distance over which diffusion is measured? What happens if the obstacles are not randomly arranged, but form a diffusion-limited aggregate? The work will describe the concentration dependence of the lateral diffusion constant of membrane proteins, such as rhodopsin in rod outer segments. It will also be applicable to the diffusion of mobile redox carriers in mitochondria, chloroplasts, and endoplasmic reticulum.