Pyruvate kinase of Neurospora crassa has been purified and some of its properties are reported. The procedure used for purification consists of five steps yielding a 90 to 95% purified protein. Preliminary sedimentation analysis yielded a sedimentation coefficient of 9.5 for this enzyme. Maximal stabilization of the enzyme is achieved in phosphate buffer; Tris buffer induces a conformational change in the enzyme leading to inactivation. Inactivation can be reversed by incubation with substrates, PEP and ADP. Preliminary kinetics studies suggest the formation of a ternary complex rather than a Ping Pong type of a mechanism.