The novel agent amphethinile is shown to inhibit tubulin assembly in vitro. This agent is capable of displacing colchicine but not vinblastine from tubulin and causes a stimulation in GTPase activity in vitro. The affinity constant for the association of this drug with tubulin has been determined (Ka = 1.3 x 10(6) M-1). It is concluded that amphethinile belongs to the class of agents which share a common binding site with colchicine on the tubulin molecule.