In recent years, we reported the isozyme shift of pyruvate kinase from the M- toward the K-type in human neuroectodermal tumors. To investigate whether this shift enables phosphorylation of pyruvate kinase in these tumors, we studied 29 different specimens of human brain tumors for endogenous pyruvate kinase phosphorylation. While in normal human brain no phosphorylation of pyruvate kinase was detected, in all brain tumors pyruvate kinase became phosphorylated. There was no correlation between the extent of the pyruvate kinase phosphorylation and the histological classification and grading or the pyruvate kinase isozyme composition of the tumors. Only pyruvate kinase type K, and not type M, served as a substrate in the phosphorylation reaction. The phosphorylation of pyruvate kinase could be completely inhibited by addition of fructose 1,6-bisphosphate, a positive effector of pyruvate kinase type K; alanine, however, a negative effector, and phospho-enol-pyruvate, a substrate in the pyruvate kinase reaction, had no effect. While pyruvate kinase type L in liver is phosphorylated by a cyclic AMP-dependent protein kinase, the incorporation of phosphate into pyruvate kinase in human brain tumors appeared to be cyclic AMP independent and occurred exclusively on serine residues.