In previous work a polyclonal B cell activator has been detected in the serum of patients with rheumatoid arthritis (RA). This activator is associated with alpha 2-macroglobulin (alpha 2M) and its activity is blocked by low-Mr trypsin inhibitors, which suggests that it may be a protease-alpha 2M complex. Here we determined the possibility of developing a routine clinical chemistry test for detection of this complex in patients' blood. We measured with chromogenic substrates the total proteolytic activity of citrated plasma and of the alpha 2M immunoabsorbed from plasma. Low-Mr substrates containing Arg were degraded much better by plasma from RA patients than by plasma from patients with other arthritides. Low-Mr substrates containing Leu, Lys, or Gly or the large-Mr substrate Azocoll were not degraded by RA patients' plasma. alpha 2M from RA patients' plasma attached to a solid-phase immunoabsorbent degraded an Arg-containing tripeptide much better than did the alpha 2M from normal donors, from patients with systemic lupus erythematosus, or from those with joint inflammation of other, "non-autoimmune" origin. Although the enzyme associated with alpha 2M in the plasma from RA patients appeared to be similar to trypsin, the differences in optimal pH, cation concentration, degradation of Lys-containing substrates, and biological activity suggest otherwise. We speculate that the alpha 2M-protease complexes are generated in the immune system and contribute to the inflammatory and autoimmune phenomena in RA.