HED10 is an autoimmune antibody (IgG) which shows considerable specificity for the single-stranded DNA poly(dT). Production of Fab fragments by papain digestion resulted in heterogeneity as judged by isoelectric focusing gels, which had a marked negative effect on crystallization. However, a single species of Fab with a pI of 7.6 could be isolated in good yield by DEAE-cellulose chromatography, and good crystals were produced by the hanging drop vapor diffusion method. The space group was P21 with cell dimensions, a = 64.2, b = 90.0, c = 42.3 A, and beta = 96.7 degrees. These crystals diffract to about 2.2 A resolution. The structure of Fab HED10 was solved by the molecular replacement method using the known structure of McPC603 and is refined to R = 27.2% at 3.0 A resolution. Fab HED10 is more extended than McPC603 and has an elbow angle (between the variable and constant domains) of 162 degrees, very similar to that observed in Fab KOL. The majority of the hypervariable regions are visible in the model.