Phosphoenol pyruvate (PEP) carboxylase has been partially purified from leaves of the C-4 tropical grass Pennisetum purpureum and shown to have allosteric properties. When initial velocities of incorporation of (14)C from NaH(14)CO3 into oxaloacetate were determined as a function of concentration of either HCO3-or Mg(2+) typical Michaelis-Menten kinetics were observed. Both Lineweaver-Burk and Hill plots were linear with values of n (interaction coefficients) of about one. Sigmoid Michaelis-Menten plots were obtained with PEP as the variable substrate. Following (NH4)2SO4 fractionation and DEAE-cellulose chromatography Lineweaver-Burk plots were concave upwards and Hill plots gave n values of two. With enzyme purified further by Sephadex G-200 chromatography Lineweaver-Burk plots were concave downwards and Hill plots gave values of n of 0.5 at low concentrations of PEP increasing to about 4 at high concentrations of PEP. Enzyme activity was modified by inclusion of glucose-6-phosphate (G6P) in the assay mixtures. When the eoncentration of G6P exceeded that of PEP, the initial velocity tended towards zero. When the concentration of G6P equalled that of PEP activity was increased. When the concentration of PEP exceeded that of G6P, the velocity approached that recorded in control samples at saturating concentrations of PEP. The rate of reaction was also increased on addition of NADH, and decreased by oxaloacetate and malate.