Crystals of pokeweed antiviral protein (PAP) from seeds ofPhytolacca americana with high diffraction ability were grown from high protein concentration (100 mg/mL) solution at high temperature (33 degrees C). The crystal structure was solved by use of molecular replacement method and refied by use of molecular dynamic method at 0.25 nm to anR factor of 18.15% with standard deviations from standard geometry of 0.001 6 nm and 2.04 for bond lengths and bond angles, respectively. Comparison with two other PAPS revealed, near the active center, a sequence- and structure-variable region, consisting of the loop connecting the fifth beta-strand with the second alpha-helix and including a proposed active residue, suggesting this loop probably to be related to difference in activity.