Pyruvate kinase purified from flounder liver in two forms, i.e. PKI and PKII, is activated by fructose 1,6 diphosphate. 2. Two or more binding sites for FDP are demonstrated for PKII, the binding to which is influenced by the levels of substrates. 3. FDP reduces or abolishes the cooperative effect of PEP. 4. FDP increases the maximal activity. 5. The inhibition observed at higher levels of ADP is not abolished by FDP.