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A reproducible polyarthritis was in-Un reproducibile polyarthritis esseva duced in rats by administering an emul-inducite in rattos per le administration sion of mineral oil and certain myco-de un emulsion de oleo mineral e certe bacteria. Associated nonarticular lesions mycobacterios. Associate lesiones nonwere observed. Pathogenesis of this ex-articular esseva observate. Le pathoperimental disease apparently involves genese de iste morbo experimental es hypersensitivity. apparentemente un question de hypersensibilitate.N 1956 THE OBSERVATION was made1 that an acute polyarthritis could I be produced experimentally in adult rats by the parenteral injection of a standard Freund-type water-in-oil adjuvant. The original inoculum contained "antigen" in the form of minced homologous muscle but it was soon found that the muscle could be omitted without affecting the incidence or severity of the arthritis. Careful observations on the clinical course of the arthritis have disclosed that it is frequently migratory or recurrent and may follow a chronic course. Furthermore a number of other interesting lesions, including a dermatitis and an iridocyclitis, have been studied both clinically and pathologically, and these will be elaborated upon. The etiology and pathogenesis of this interesting series of reproducible lesions is not as yet understood. Two principal hypotheses have been considered throughout the investigations. The first is that the arthritis and other lesions are due to the activation of an infectious agent or agents in these laboratory animals. Particular attention has been directed toward the potential role of the pleuropneumonia-like organisms ( PPLO ) or Streptobacillus moniliformis, since each has been associated with an arthritis in rats. Our studies have tended to exclude either of these organisms from further serious consideration as causative factors. The second postulate is that the lesions may be pathologic expressions of an immunologic (allergic) disease. The antigen in this situation may be some component of the tubercle bacilli incorporated into the adjuvant or perhaps some constituent of the tissues into which the adjuvant is injected. A number of features of the disorder favor the immunologic hypothesis.