Purified tubulin fully liganded to GDP at the exchangeable nucleotide binding site has been prepared by a new direct nucleotide exchange procedure. This normally inactive GDP-tubulin is driven to assemble into microtubules by the binding of the antitumor drug taxol or its more soluble side-chain analogue Taxotere in Mg(2+)-containing buffer, and it disassembles by cooling the solution. Therefore this ligand-induced equilibrium microtubule assembly system dispenses with the requirement of a gamma-phosphate-metal cation ligand bound at the nucleotide site for tubulin to be active. GDP-tubulin can also form characteristic pseudo-ordered aggregates of double rings. These aggregates dissociate upon warming or by addition of GTP. Back-substitution of the nucleotide gamma-phosphate permits glycerol-induced assembly without taxol and reduces the critical protein concentration required for drug-induced microtubule assembly by a factor of 2.6 +/- 0.1. The ligand-induced assembly is maximal at taxol or Taxotere concentrations equimolar with tubulin, and both drugs bind to assembled tubulin with a stoichiometry of 0.99 +/- 0.04 ligand per alpha beta dimer. Taxotere apparently competes with taxol for the same binding site, with 1.9 +/- 0.1 times larger effective affinity. Similarly, the Taxotere-induced assembly of GDP-tubulin or GTP-tubulin proceeds with a critical protein concentration 2.1 +/- 0.1 times smaller than with taxol.