Protein-protein association is clearly a very important process in biology, including events such as signal transduction, immune response, and transcription. Protein-protein association rates are in the range of 103 to 109 M-l S-1, with those in the diffusion-limited regime having rates of .106 M-1 s. One well-characterized protein-protein system, barnase-barstar, associates at a rate of _ 108 M-1 sl, falling well into the diffusion-limited regime (Schreiber and Fersht, 1996, and references therein). There are other factors that help to identify a system as being diffusion limited, which are that the association rates are influenced by solvent viscosity, ionic strength, temperature, and diffusional environment. The barnasebarstar system has such a high rate of association because of the contribution of electrostatics (Schreiber and Fersht, 1996). Wild-type barnase has a net charge, at pH 8, of +2, and that of barstar is -6. In this issue, Gabdoulline and Wade (1997) describe the application of their effective charge method (Gabdoulline and Wade, 1996) in a simulation of the diffusional encounter of barnase and barstar. Protein-protein association still represents a significant challenge for computational chemistry, even though substantial advances have been made in the development of theory for these problems. Molecular dynamics methods are generally too time consuming and demanding, especially if explicit solvent is included. Most computational work on protein-protein as-