X-ray crystallographic analysis was performed and several phenylahistin derivatives were synthesized to elucidate the structural components necessary for the anti-microtubule activity of phenylahistin. We primarily focused on the unique isoprenylated dehydrohistidine structure. Our results showed that a uniplanar pseudo-three-ring structure formed by the hydrogen bonding of diketopiperazine and imidazole rings is important for the anti-microtubule activity of phenylahistin. (C) 1999 Elsevier Science Ltd. All rights reserved.