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We have expressed six previously cloned isoforms of human microtubule‐associated tau protein in Escherichia coli and purified them to homogeneity in a biologically active form. They range from 352 to 441 amino acids in length and differ from each other by the presence of three or four tandem repeats in the carboxy‐terminal half and by the presence or absence of 29 or 58 amino acid inserts in the amino‐terminus. When mixed together they gave a set of six bands on SDS‐PAGE gels with apparent molecular weights of 48–67 kd and with a characteristic pattern of spacings. Four of these bands aligned with the major tau bands found in adult human cerebral cortex following perchloric acid extraction and alkaline phosphatase treatment. They consisted of isoforms with three repeats and no insertions, four repeats and no amino‐terminal insertions and three‐ and four‐repeat containing isoforms with the 29 amino …