High activities of L-threonine and l-serine dehydratases were maintained in spontaneous hepatoma of mice strain CBA as distinct from transplantable hepatoma. The initial rate [v] versus substrate concentration [S]0 curves had complex shapes for the enzymes from the liver tissue of healthy animals (especially after extraction of the enzymes by means of buffers with low concentration of K+). Kinetic patterns of l-threonine and L-serine dehydratases from hepatoma were distinct from those of normal mice liver tissue. The shape of [v] versus [S]0 plots was altered in response to AMP (1.10(-3) M). Contrary to the enzymes from normal tissue, dehydratases from hepatoma did not alter the shape of [v] versus [S]0 curves in response to AMP. The enzymes from hepatoma were apparently desensitized with respect to their possible allosteric effector. Threonine dehydratases of normal mice liver were also dissimilar as compared with the enzymes from hepatoma in the affinity of the apoenzyme to pyridoxal 5'-phosphate. This affinity was 3-fold lower for threonine dehydratase from hepatoma as compared with the enzyme from liver tissue.