Two different tubulin-assembly-promoting proteins were isolated from porcine brain microtubule protein. Following a heat step performed on microtubule protein, the thermal-stable proteins were fractionated by chromatography on phosphocellulose and Sepharose 4B. Two highly purified associated proteins were obtained. One resembles the previously described MAP2 protein (polypeptide molecular weight approximately 300,000), the other a mixture of four or five polypeptides previously described as tau protein (molecular weights between 55,000 and 70,000). Both proteins stimulate the polymerization of pure brain tubulin into microtubules with comparable activity. The resulting microtubules were characterized by electron microscopical analysis. Microtubules polymerized in the presence of MAP2 protein show typical side projections, which are conspicuously absent in microtubules assembled in the presence of tau protein. The latter microtubules show smooth surfaces. Some biochemical similarities and differences between the two different microtubule-associated proteins are discussed.