We demonstrate here that brain purified tubulin can be dissociated into alpha and beta subunits at pH > 10 and that the subunits can be separated by using the Triton X-114 phase separation system. After phase partition at pH > 10, alpha tubulin but not beta tubulin behaves as a hydrophobic compound appearing in the detergent rich phase. After three extractions of the alkaline aqueous phase with Triton X-114, about 90% of the alpha tubulin was recovered in the detergent rich phase. The hydrophobic behavior observed for alpha tubulin after its dissociation at pH 11.5 was not due to an irreversible change of the protein, because when the detergent rich phase containing alpha tubulin was diluted with a buffer solution at pH 7.3 and the solution allowed to partition again, alpha-tubulin is recovered in the aqueous phase. The detergent in the aqueous phase of the alpha and beta tubulin preparations can be removed up to 90% by 12 h dialysis. The alpha and beta subunits of tubulin from kidney and liver behave, in this phase separation system, like those of brain tubulin.