The binding of transcription factor ATF-1 to DNA contributes to gene expression and regulation of cell growth. Antibody Mab41.4, raised against ATF-1, and its derivatives Fab41.4 and scFv41.4 inhibit specific DNA binding in vitro and induce apoptotic death of tumor cells in vivo. Structural studies of Fab41.4 were performed to gain insight into the mechanism of action of this potentially therapeutic antibody. The optimal conditions for crystallization of Fab41.4 were determined. Crystals were needle-like in appearance, displayed C2 space-group symmetry and diffracted to a resolution of 1.6 A. The unit-cell parameters were determined to be a = 186.64, b = 40.22, c = 55.58 A, alpha = gamma = 90, beta = 96.93 degrees. The data set was 97.7% complete. Molecular replacement was performed, resulting in an R value of 44.6%.