A solution of an apparently nonimmunogenic synthetic compound, N,N-dioctadecyl-N',N'-bis(2-hydroxyethyl) propanediamine (CP-20961), suspended in mineral oil or olive oil (50 mg/ml), induced an acute, as well as a chronic, polyarthritis when single intradermal injections (0.2 ml) were made in the tail or hindpaw of Lewis rats. The polyarthritis was morphologically almost indistinguishable from classic adjuvant arthritis induced by Freund's complete adjuvant (FCA), a disease generally thought to be the result of a delayed hypersensitivity reaction to a constituent(s) of the injected tubercle bacilli. The disease induced by CP-20961 and that induced by Freund's complete adjuvant followed the same time course and almost identical pattern of development of clinical and histopathologic features. Like the classic adjuvant arthritis, CP-20961 induced arthritis is suppressed by an immunosuppressive agent (cyclophosphamide) or an antiinflammatory drug (phenylbutazone). The alkyldiamine (CP-20961) was found to be a potent adjuvant; a dispersion or a solution of the compound in mineral oil administered intraperitoneally enhanced the development of both the cell-mediated and the humoral immune responses to EL4 cells in the rat. These findings suggest that the immunogen responsible for the development of adjuvant arthritis is endogensou, e.g., a constituent of host tissue, a viral protein, or some complex of the two.