In living organisms microtubules exist as highly labile or very stable structures. In higher eucaryotes, the main, and probably the only, physiological effectors capable of triggering microtubule transtions between lability and stability, are specialized proteins.In the present paper, general methods to assay and isolate these effectors are proposed. The properties of a particular protein (STOP protein) are shown. Finally the potential significance of such effectors in the generation of organelle movement and microtubule spatial self organization are discussed.