Sigma (Software of Imagery and Graphics for Molecular Architecture) is a software package designed for visualization and exploration of three-dimensional (3D) molecular electron microscopy data. It comprises a text and two graphic modes. In the text mode, a language allows the writing of scripts for repetitive tasks. The two graphic modes are based on the multiwindow environment with a mouse as pointer. The first graphic mode is dedicated to vectorial objects and the second to pixel/ voxel objects, The data imported by SIGMA have various origins such as 3D reconstruction volumes, Protein Data Bank data, or geometrical objects like spheres, cubes, or cylinders. These 3D objects are interactively rotated, translated, and rendered with SIGMA. This software, written in C language, runs on Silicon Graphics workstations using the X-Window plus Motif environment. The specific 3D routines are based on the Silicon Graphics GL library. (C) 1996 Academic Press, Inc.