Fractions A (salted out by ammonium sulphate between 21-30% saturation), and fractions B (salted out between 51-70% saturation) of pyruvate kinase (EC 2.7.1.40.) corresponding respectively to pyruvate kinase types L and M2 from rat liver and Morris hepatoma 7777 were purified by an affinity chromatography on Blue Sepharose CL-6B. Peaks of inactive proteins were eliminated and the enzyme fractions bound biospecifically to the gels were eluted by free ADP. The molecular mass of purified hepatoma pyruvate kinase fraction B was smaller than that of liver pyruvate kinase fraction B. Morris hepatoma pyruvate kinase fraction B represented a variant of type M2, characterised by greatest affinity to 2-phosphoenolpyruvate as a main substrate and different sensitivity to low-molecular effectors in comparison with types L from both liver and hepatoma and in comparison with type M2 from normal rat liver. Only this hepatoma fraction B showed a tumour specific sensitivity to L-cysteine and was insensitive to normal signal molecules i.e. to ATP and fructose-1,6-diphosphate which influence liver pyruvate kinase activity. L-Cysteine inhibited the tumour fraction B of pyruvate kinase by decreasing its Vmax and increasing the Km values in relation to 2-phosphoenolpyruvate.