A new procedure for the purification of cytochrome b5 and NADPH-cytochrome c reductase from rat liver microsomes is described. This procedure is a preparative application of a small-scale procedure reported earlier, and is suitable for purifying both enzymes simultaneously starting from several hundred grams or more of rat liver. The method is applicable to liver microsomes of other animals. The recovery of cytochrome b5 and NADPH-cytochrome c reductase from microsomes was 35–45%, and the homogeneity of purified preparations was confirmed by sedimentation analysis and disc electrophoresis. The high purity of the preparations obtained by the previous small-scale procedure was also confirmed. Purified NADPH-cytochrome c reductase was reducible by either NADPH or NADH, and had a very weak NADH-linked reductase activity.