Homogeneous phosphoribulokinase (PRK ; ATP : D-ribulose-5-phosphate 1 -phosphotransferase, EC 2.7.1.19) was isolated from wheat leaves with a specific activity of 15 pkat mg-' protein. The purification included ammonium sulfate cuts, isoelectric precipitation, and hydrophobic and affinity chromatography on pentylagarose and Blue Sepharose CL 6B, respectively. Gel filtration of the purified enzyme yielded a 83000 Da protein. Subunits of about 42000 Da were estimated from sodium dodecyl sulfate-polyacrylamide gels. Wheat leaf PRK was stable for at least four weeks when stored at 4" C. Saturation curves for ribulose 5-phosphate (RuSP) and ATP followed Michaelis-Menten kinetics ( K , values : K, .,,, = 50-80 pM ; K, .,, = 70 pM). The saturation curve for MgCI, was sigmoidal (half-maximal velocity <0.5 mM). The affinity for RuSP, ATP and Mg2+ was not affected by pH changes comparable to pH shifts in the stroma. In contrast to chloroplast fructosebisphosphatase (Zimmermann et al. 1976, Eur. J. Biochem. 70, 361-367) the affinity for ligands remained unchanged in the dithiothreitol-activated and in the non-activated state. The activity of PRK was increasingly sensitive to inhibition by 3-phosphoglyceric acid with decreasing pH below pH 8.0.