Rats receiving intravenous injection of human albumin (4 mg/rat) once biweekly developed liver fibrosis. The lesion seemed to have little relation to hepatocellular injury. The incidence of liver fibrosis increased with the length of immunization, ranging 80%-86% after 30-60 days. The whole process of experimental liver fibrosis may be divided into three phases. The first is the initial stage of fibroplasia (from the first day to the time of 15 days after start of immunization). In this phase, Ito cells were activated and collagen type I and type III began to increase. The second is the activated stage of fibroplasia (from the 15th to the 60th day after the beginning of immunization), in which collagen type I and type III reached the maximum and myofibroblasts as well as 'transitional cells' proliferated with deposition of collagen fibers. The third phase is the stage of post-fibroplasia (the period after elimination of immunization). Collagen type III diminished gradually while collagen type I remained increasing. Our findings indicated that fibroplasia occurs at the very beginning of liver injury. This suggest that treatment of liver fibrosis must be considered simultaneously with treatment of acute hepatitis.