A third-generation synchrotron source was used to collect data from crystals with a very large unit cell. There was an increase in the effective resolution of the data from 5 to 3.5 A. Data were collected on crystals of HK97 mature empty capsids, space group P2(1), with unit-cell parameters a = 580, b = 625, c = 790 A, beta = 90.0 degrees. Like other crystals with very large unit-cell dimensions, the intensity falls off rapidly as a function of resolution, with a precipitous drop beginning at 9 A resolution. Synchrotron data from these crystals were previously observed at the CHESS F1 beamline to about 3.5 A resolution, but the intensities could not be accurately measured beyond 5 A. In experiments conducted at the Advanced Photon Source (APS) beamline 14-BM-C, data from identical crystals could be processed to a resolution of 3.5 A. The lifetime of the crystals in the beam was increased from one exposure per crystal volume to between four and eight exposures per crystal volume. More than 500 images were collected in two trips, allowing the extension of the resolution of the data set and the structure determination to 3.5 A resolution. Factors in the increased resolution may include X-ray flux, beamline geometry and low background scatter. These results suggest that other crystals with large unit cells and pronounced intensity falloff with resolution may benefit from the use of this or similar beamlines.