The binding of Glyceraldehyde−3−phosphate dehydrogenase (GAPDH; E.C. 1.2.1.12) to microtubules causes the microtubules to assemble into large bundles. This bundling can be considered as a further step in the assembly of supramolecular structures.The rate of bundle formation, after addition of GAPDH to preformed microtubules, is not dependent on the GAPDH concentration and reflects bundling kinetics.Bundle disassembly can be studied by the addition of 1 mM adenosine 5’−(&bgr;, −imidotri−phosphate) (AMPPNP) to bundled microtubules, and is extremely fast.Bundling reduces the rate of association of tubulin dimers to microtubules, as well as the dissocation from the microtubles. Both rates are reduced to the same extent. This is in agreement with the fact that the critical concentration of tubulin is practically not influenced by the binding of the enzyme.Adding microtubule associated proteins (at I=0.1 M) does not appreciably influence the affinity for GAPDH, but reduces bundle formation possibly for sterical reasons.