Both soluble abnormally phosphorylated tau and tau in paired helical filaments are abnormally glycosylated. Abnormally modified tau is incompetent in promoting the assembly of microtubules. Dephosphorylation of insoluble tau with various phosphatases restores differentially the biologic activity of tau in promoting the assembly of microtubules. Deglycosylation of tau enhances the above-mentioned activity caused by dephosphorylation although deglycosylation alone has no significant effect on restoring the biological activity of tau. These data suggest that abnormal phosphorylation of tau might be the direct factor for its deficient function whereas glycosylation might be an indirect one by affecting the structure of tau; that protein phosphatases might arrest and reverse the lessions in Alzheimer brain.