In a recent paper Main and Woolfson (1963) proposed a technique for obtaining accurate lattice parameters from zero-level Weissenberg photographs. The method derives refined cell constants by applying least-squares analysis to α1 — α2 separations of high-angle reflections. For theory of the Main-Woolfson method, the reader is referred to their original paper. We have written a Fortran code, following the Main-Woolfson scheme, and have tested it by obtaining accurate lattice parameters of gaylussite which are then used to index its powder pattern. The cell parameters of gaylussite are also obtained independently from its powder pattern using a least squares program written by Argonne National Laboratory.