The present study was designed to prepare and characterize subcellular fractions from the intestinal mucosa of the Northern pike (Esox lucius), with special emphasis on the preparation of a microsomal fraction suitable for studying xenobiotic metabolism. The purity of the different fractions obtained by differential centrifugation, as well as the recovery of different organelles, was determined using both enzyme markers and morphological examination with the electron microscope. The subcellular distributions of several enzymes involved in drug metabolism (NADPH-cytochrome c reductase, NADH-ferricyanide reductase, epoxide hydrolase activity towards both cis- and trans-stilbene oxide as substrates, and glutathione transferase) were also examined. The subcellular distributions obtained here for drug-metabolizing and marker enzymes closely resembled those reported for rat and pike liver. The microsomal fraction obtained contained about 50% of the total endoplasmic reticulum. This fraction was relatively free of nuclei, mitochondria, Golgi, peroxisomes and cytosol, but relatively heavily contaminated with lysosomes and fragments of the plasma membrane. Within the limitations discussed, the subfractions prepared here are suitable for further characterization of drug-metabolizing systems in the intestinal mucosa of the Northern pike, as well as for other studies with this tissue.