All liver tumors induced in rats by AAF or DAB, but only one out of 29 tumors induced by ethionine, exhibited new isozymes of aldehyde dehydrogenase. These new isozymes, which differed from those of normal liver with regard to activity, pI, stability, cellular distribution, and coenzyme specificity, were not found in fetal or regenerating rat liver, nor in spontaneous liver tumors of the mouse or mastomys, nor in several other tumors appearing in rats bearing AAF-induced liver tumors, nor in any of several transplanted Morris hepatomas. The activity of the new isozymes did not correlate with histological criteria of differentiation, and the new forms were sometimes detectable in the liver of AAF-fed rats in areas where no tumor was macroscopically or histologically visible.