Previously we partially purified a novel protein kinase which phosphorylated tau and formed a paired helical filament (PHF) epitope. In this paper we show that the kinase fraction contains a protein kinase activity recognizing serine/threonine proline sequence. The kinase phosphorylated tau at the tau-1 site previously reported as one of the phosphorylation sites on PHF by other groups. The kinase also phosphorylated extraordinarily insoluble portion located on C-terminal region of tau in PHF. It is worth considering that tau phosphorylated by this kinase activity is incorporated into PHF.