Binding immunoglobulin protein reduces gross pathology of synovial membrane transplants. Pieces of human synovial tissue from patients with rheumatoid arthritis were transplanted into severe combined immunodeficient mice. After successful engraftment, binding immunoglobulin protein (BiP) (10 μg/animal) or human serum albumin (HSA) (10 μg/animal) were administered intravenously, and 12 days later the tissue was removed for immunohistological examination. Representative figures of haematoxylin-stained tissue removed from (a) HSA-treated mice or (b) BiP-treated mice. Transplant samples were scored by (c) Rooney and colleagues' and (d) Koizumi and colleagues' histological criteria. Results are the mean and standard deviation of 15 animals in each group. *P < 0.02 in a comparison of BiP-treated and HSA-treated samples.