Naccess is a stand alone program that calculates the accessible area of a molecule from a PDB (Protein Data Bank) format file. It can calculate the atomic and residue accessiblities for both proteins and nucleic acids, and is available for free from this site for researchers at academic and non profit-making institutions. We are now able to distribute the software for a fee to Industrial and Profit-making institutions. It runs on most Linux/Unix systems including cygwin under windows.