Phosphoinositol kinase was purified from suspension-cultured rice (Oryza sativa L,) cells, The apparent molecular mass of the rice enzyme was estimated to be 58 kDa by SDS-PAGE and 234 kDa by Toyopearl HW-55S gel filtration, indicating that it is a tetrameric enzyme. The enzyme absolutely required Mg2+ for the activity, but Mn2+ and Ca2+ did not affect it. In addition, this kinase phosphorylated inositol monophosphate to phytate, Judging from these data, rice phosphoinositol kinase was concluded to be a different enzyme distinguished from the other plant phosphoinositol kinases.