The SURFNET program generates molecular surfaces and gaps between surfaces from 3D coordinates supplied in a PDB-format file. The gap regions can correspond to the voids between two or more molecules, or to the internal cavities and surface grooves within a single molecule. The program is particularly useful in clearly delineating the regions of the active site of a protein. It can also generate 3D contour surfaces of the density distributions of any set of 3D data points. All output surfaces can be viewed interactively, along with the molecules or data points in question, using some of the best-known molecular modeling packages. In addition, PostScript output is available, and the generated surfaces can be rendered using various other graphics packages.