MRL-lpr/lpr (MRL/l) mice spontaneously develop a disease that is characterized by glomerulonephritis, diffuse vasculitis, and arthritis associated with high levels of autoantibodies that include IgG rheumatoid factor (RF). To define the immunopathogenic mechanisms that lead to the development of extravascular lesions such as arthritis, we implanted a tissue cage subcutaneously in arthritic MRL/l mice and compared components of the tissue cage fluid, which resembles the extravascular fluid, with those of sera. When compared with those of sera, tissue cage fluids from arthritic MRL/l mice had similar levels of RF and one-third the amount of C1q immune complexes. In contrast, anti-DNA activities in tissue cage fluids corresponded to only 10% of the serum activities and, most strikingly, nephritogenic retroviral gp70-anti-gp70 immune complexes were almost undetectable in tissue cage fluids. This was also the case for another strain of autoimmune mice, (New Zealand black X New Zealand white)F1 mice, although they did not produce RF. The absence of gp70 immune complexes in tissue cage fluids could be due to markedly limited diffusion of gp70 antigen in these fluids. These results strongly suggest that serum proteins, including autoantigen and autoantibodies, appear in extravascular fluid in a selective manner, depending on their size and charge. Their specific properties in sera or extravascular fluid could partly account for the different manifestations of vascular and extravascular lesions observed in autoimmune mice.