Indoxyl acetate esterase was demonstrated in rat liver by an azo-indoxyl method at the fine structural level. The present study on the intracellular localization of esterase activity suggests that the esterase activity in centrifugal fractions might not be the contamination of the microsomal fraction. 10-3 M sodium fluoride completely suppressed the staining of the cytoplasmic membranous structure, but did not suppress the stain of the droplets, exhibiting the so-called peribiliary activity of type A esterase.