A microsome preparation procedure using polyethylene glycol(PEG )is reported. The liver homogenate of rats is aggregated by polyethylene glycol-6000,and centrifuged at high speed two times.The obtained pellet is the microsomal fraction. Compared with the ultracentrifugation method,this procedure of using PEG to prepare microsomes greatly eliminated the need for an ultracentrifuge and reduces the time necessary for isolation of this fraction. It is simple and feasible.