This article describes a method of immunization that produces chronic serum sickness in rats within a relatively short time. Fisher rats, which were immunized subcutaneously three times with bovine serum albumin (BSA) in adjuvant, responded with high titers of antibodies to BSA. 2 weeks after the third subcutaneous immunization, daily increasing amounts of BSA were injected either intraperitoneally or intravenously. When an intravenous dose of 2mg of BSA was reached, the rats were given daily intravenous injections of BSA for several weeks. This procedure, which avoided death from anaphylaxis, induced severe proliferative glomerulonephritis in all the rats and produced deposition of antigen-antibody complexes in many other organs besides the kidney. This highly reproducible model of experimental chronic serum sickness in inbred animals may have applications for the study of the mechanisms of immune complex disease.