S ystemic administration of an aqueous suspension of group A streptococcal cell wall fragments to susceptible rats induces acute and chronic polyarthritis, as well as noncaseating hepatic granulomas. To gain insight into the role of the thymus in the pathogenesis of this experimental model, pathologic responses and cell wall tissue distribution were compared in congenitally athymic rats (rnu/rnu) and their euthymic littermates (NIH/rnu). Within 24 h, both rat strains developed acute arthritis, characterized by polymorphonuclear leukocytic exudate in the synovium and joint spaces. This acute process was maximal at day 3 and gradually subsided. Beginning 2-3 wk after injection, the euthymic, but not the athymic, rats developed the typical exacerbation of arthritis, characterized by synovial cell hyperplasia with villus formation and T helper/inducer lymphocyte-rich mononuclear cell infiltration. This process eventually resulted in marginal erosions and destruction of periarticular bone and cartilage. Parallel development of acute and chronic hepatic lesions was observed. Bacterial cell wall antigen distribution and persistence were similar in the athymic and euthymic rats. Cell wall antigens were demonstrated in the cytoplasm of cells within subchondral bone marrow, synovium, liver, and spleen, coincident with the development of the acute lesions, and persisted in these sites, although in decreasing amounts, for the duration of the experiment. Our findings provide evidence that the acute and chronic phases of the experimental model are mechanistically distinct. The thymus and functional thymus derived-lymphocytes appear not to be required for the development of the acute exudative disease but are essential for the development of chronic proliferative and erosive disease. Induction of disease is dependent upon cell wall dissemination to and persistence in the affected tissues.