This study isolates a heat-stable protein essential for microtubule assembly, designated as tau (τ). Tau is present in association with purified tubulin from porcine brain and is separated from tubulin using ion exchange chromatography on phosphocellulose. In the absence of tau, tubulin exists as a 6S dimer (α and β tubulin) that cannot form microtubules in vitro. However, the addition of tau restores the tubulin's ability to form microtubules. Under non-polymerizing conditions, tau converts the 6S dimer into 3S subunits, which initiate the assembly of tubulin at intermediates in tubule formation. This suggests that tau acts on the 6S tubulin dimer, activating it for polymerization. The unique ability of tau to restore normal in vitro microtubule assembly indicates that it is a major regulator of microtubule formation in cells. The study also explores the properties and interactions of tau with tubulin, providing insights into the regulatory mechanisms of microtubule assembly.This study isolates a heat-stable protein essential for microtubule assembly, designated as tau (τ). Tau is present in association with purified tubulin from porcine brain and is separated from tubulin using ion exchange chromatography on phosphocellulose. In the absence of tau, tubulin exists as a 6S dimer (α and β tubulin) that cannot form microtubules in vitro. However, the addition of tau restores the tubulin's ability to form microtubules. Under non-polymerizing conditions, tau converts the 6S dimer into 3S subunits, which initiate the assembly of tubulin at intermediates in tubule formation. This suggests that tau acts on the 6S tubulin dimer, activating it for polymerization. The unique ability of tau to restore normal in vitro microtubule assembly indicates that it is a major regulator of microtubule formation in cells. The study also explores the properties and interactions of tau with tubulin, providing insights into the regulatory mechanisms of microtubule assembly.