(A) The effects of phosphate, chloride, nitrate, pyruvate, malate, succinate and glutamate ions on the kinetics of yeast phosphoglycerate kinase (ATP: 3-phospho-D-glycerate 1-phosphotransferase, EC 2.7.2.3) were studied with MgATP2- and 3-P-glycerate as variable substrates. Three types of patterns were obtained: (1) Nitrate, succinate, malate and glutamate ions, strictly noncompetitive versus both the substrates. (2) Phosphate and chloride ions, noncompetitive versus MgATP2- and mixed versus 3-P-glycerate. (3) Pyruvate ions, being very weak inhibitors, competitive with MgATP2- and noncompetitive with 3-P-glycerate. (B) Based on experiments with simultaneous inhibition by various combinations of two anions the following suggestions were made: The type 1 anions presumably bind to a site outside the active centre. These ions appear to bind to the enzyme independently of type 2. The latter also appears to include sulfate ions, which are competitive versus both the substrates as well as versus the phosphate and chloride ions. Sulfate and phosphate ions are electronically similar, but show different inhibition patterns, presumably due to various effects on the protein conformation. Type 3 inhibition exerted by pyruvate ions was shown earlier for 1-anilino-8-naphthalenesulfonate and salicylate ions, but as these two anions are supposed to bind to the adenine binding pocket of the catalytic centre, the results indicate that pyruvate ions might preferably compete with the nucleotide substrate for the polyphosphate binding site.