Abstract A preparation of l -phenylalanine ammonia-lyase (EC 4.1.3.5.) from soybean ( Glycine max L. cv. Kanrich) showed negative cooperativity with respect to l -phenylalanine and competitive inhibition by d -phenylalanine. A two-protomer partially concerted model for inhibition kinetics is described. If cooperativity is associated with ligand binding but not k cat , plots of v against log [S] at constant [I] are symmetrical. Such curves may be fitted by graphical or iterative least-squares methods. The experimental results conform to this restricted model. The three-substrate and three-inhibitor dissociation constants were estimated by a stepwise procedure. For substrate only the first and second dissociation constants were 12 and 78 μ m , respectively, with a symmetry point value of 30.5 μ m . To a first approximation, site occupancy determines the cooperativity. As d - and l -phenylalanine produce equivalent effects, they are assumed to pack into the same induced space. As ligand binding at one site has little influence on the relative d : l binding at the other and does not influence k cat , cooperativity probably reflects changes in regions remote from the active site such as the interface between the protomers. The regulatory range in [S] of the enzyme in vivo may be indicated by the linearity range of the semilog plot for the isolated enzyme. The observed range corresponds to a 100-fold change in [S] compared to a 10-fold change for Michaelis-Menten kinetics.