We implement a graphical user interface in an X-window/UNIX environment to compute and display the incoherently averaged Fourier transforms of electron images of single particles embedded in ice and the simulated contrast transfer function with or without envelope functions. This interface provides an easy and efficient operation for the determination of defocus value and the evaluation of the extent of Fourier amplitude falloff. This computational procedure is crucial for prescreening image data and performing image correction of contrast transfer function in high-resolution three-dimensional reconstruction of single particles. (C) 1996 Academic Press, Inc.