Abstract The distribution of polar dyes in the subcellular fractions of rat liver was studied after 3′-Me-DAB administration. In experiments with stomach tube administration the polar dye concentration, per unit weight of protein, was higher in the supernatant fraction than in two microsomal subfractions soluble or insoluble in DCA. These experiments covered a period of 4–40 hr after a single dose of 25 mg of the dye. Similar results were obtained in feeding experiments. For the supernatant fraction, a binding maximum appeared one week after the beginning of the feeding, while for the two microsomal sub-fractions it appeared two weeks after the beginning. A rather high concentration of protein-bound dye was observed in the microsomal ribonucleoprotein particles over the whole experimental period. This observation was discussed with respect to the tumor induction mechanism.