The assembly of chick brain microtubule protein in a NaCl-supplemented buffer has been examined with respect to nucleation and the subsequent elongation as a function of the nucleotide (GTP vs 2'dGTP), and the protein and nucleotide concentrations. The kinetics suggest that unassembled tubulin can exist in two conformational states (termed Tu1,GTP and Tu2,GIP when GTP is bound to the exchangeable site), with Tu1,GTP contributing to nucleation and Tu2,GTP participating in elongation. The extent of self-nucleation is proposed to be determined, in part, by the rate constant governing this conformational change. This analysis contrasts with that of earlier studies, which concluded that the number of subunits interacting to form an effective nucleus could be estimated from the dependency of self-nucleation on the initial concentration of unassembled tubulin.