Purified high-molecular-weight microtubule-associated protein 2 (MAP2) labelled with [32P]-phosphate has been obtained and used as a marker to study the incorporation of this protein into microtubules at steady state in vitro. The incorporation kinetics of protein MAP2 show two different mechanisms of addition of this protein into microtubules. A fast incorporation, which proceeds essentially independently of microtubule assembly, indicates its addition into binding positions which may be available along the structure. Once the microtubule has been saturated with the protein, its incorporation proceeds from one end of the microtubule, in a polymerization-dependent manner, at a slower rate. Loss of the protein from microtubules at steady state proceeds fundamentally at the opposite end.