Using a new C3a-radioimmunoassay, we could show a significantly increased complement activity in patients with active rheumatoid arthritis. The C3a-plasma level in rheumatoid patients was 529 ng/ml, whereas in patients with psoriatic arthropathy it was 164 ng/ml, in patients with noninflammatory muscular pain syndromes 170 ng/ml and in healthy blood donors we found a mean level of 221 ng/ml. The reproducibility was high, therefore it seems that the possibility to calculate the risks of complement-activating immunological disorders can be increased by using the C3a-radioimmunoassay. On the other hand the C3a-increase in rheumatoid patients may be an expression of the immunological activity of the disease.