The levels of alpha 2-macroglobulin and tissue inhibitor of metalloproteinases (TIMP) were determined in paired serum and synovial fluid samples from rheumatoid arthritis patients. Serum levels of alpha 2-macroglobulin (mean 1580 +/- 340 mg/l) were always higher than the corresponding synovial fluid although the spread of synovial fluid levels was broad (mean 520 +/- 340 mg/l). Tissue inhibitor of metalloproteinases was always higher in synovial fluid (mean 4.59 +/- 1.8 mg/l) than serum (mean 1.3 +/- 0.39 mg/l) indicating that local synthesis occurred within the joint.