The PIC system is an integrated package of image processing software written in Fortran and G. Throughout its 16 years of continuous development, PIG has been designed for the processing of electron micrographs with emphasis on the particular requirements for structural analysis of biological macromolecules. PIG has been implemented on successive generations of Digital Equipment Corporation dedicated minicomputers and workstations, The latest version, PIG-III, runs on Alpha workstations and represents a substantial upgrading in functionality compared with PIG-II, the VAX version previously described (B. L. Trus et al. (1992), Scanning Microsc. Suppl. 6, 441-451), PIG now possesses an X-windows menu-driven graphical user interface that many be utilized from both local or remote X-window terminals or workstations. Other new features include dynamic memory allocation; an open systems approach to interfacing with other image processing software packages; interface options with visualization software; and programs to reconstruct three-dimensional density maps of both helical filaments and free-standing particles with and without symmetry. (C) 1996 Academic Press, Inc.