Purified calf brain tubulin has been induced to self-aggregate in vitro into paracrystalline structures by the anti-cancer drug vinblastine. The size, shape, optical birefringence, and drug stoichiometry of these tubulin-vinblastine paracrystals are similar to those of paracrystals formed in vivo. Similar structures can be formed with vincristine and desacetylvinblastine, but not with colchicine or podophyllotoxin.