Pyruvate dehydrogenase (PDH) activity has been evaluated with respect to normal liver in 3 lines of Morris hepatomas (H), i.e. the highly differentiated H 9618A, the well differentiated H 44 and the poorly differentiated H 3924A. Assays of both total (PDHt) and active (PDHa) form show a progressive decrease of enzyme activity going from liver to the H 3924A. PDHa better correlates with the degree of hepatoma differentiation than does PDHt. Further enzyme analysis has been achieved in partially purified extracts from liver and H 3924A. The possible implications of such an enzymatic variation are discussed.