The esterases contained in 32 mouse tissues and organs have been analyzed using the zymogram technique. More than ten different esterases have been resolved and partly characterized by their substrate and inhibition specificities. The esterases compose a family of distinct enzymes with overlapping specificities, and the total activity in any tissue is the sum of the activities of the particular esterases in that tissue. Characteristic similarities and differences among the esterase patterns from different tissues were noted and discussed. The G band is apparently a cholinesterase as shown by its sensitivity to inhibition by eserine at 10–4 M. The small effect of age, sex, genotype, and nutrition upon the esterase pattern is contrasted with the striking changes occurring during the differentiation of organs. The greatest change not associated with differentiation was observed in blood after 5 days of starvation. Fractionation of liver homogenates by centrifugation showed esterase to be concentrated primarily in the microsome fraction. Measurements were made of the quantitative distribution of esterase activity among the electrophoretically resolved bands from each of the centrifugal fractions of liver homogenates. The pattern of esterases was very similar in each of the centrifugal fractions.