The levels of alpha1-proteinase inhibitor-elastase and alpha 2-macroglobulin (alpha 2M)-proteinase complexes were measured in synovial fluids from arthritis patients by use of specific immunosorbent assays. Both types of proteinase inhibitor-proteinase complexes were significantly correlated with each other as well as with the total neutrophil count in synovial fluids of rheumatoid arthritis patients but were discordant in synovial fluids of patients with osteoarthritis. One synovial fluid sample showed active (inhibitory) alpha 2M as well as active collagenase. We purified alpha 2M from pooled synovial fluids obtained from patients with rheumatoid arthritis. This alpha 2M retained approximately 90% of its proteinase binding (inhibiting) capacity, compared with that of normal plasma alpha 2M. We found no evidence that alpha 2M was inactivated by means other than proteinases.