Levels of C3a in plasma are currently measured by a competitive inhibition radioimmunoassay (RIA) in which 125I-C3a is used as a tracer. In this paper, we describe a modification of this RIA: 125I-C3 instead of 125I-C3a is used. The lower limit of detection of this modified RIA is 6 ng of C3a per ml of plasma (i.e. 0.66 nmol/l). This RIA, performed with polyclonal anti-C3a antibodies coupled to a solid phase, appeared to be 30 times more sensitive compared with an RIA in which a monoclonal antibody against C3a is used. In vitro activation of the complement system in serum by aggregated IgG, zymosan, and cobra venom factor resulted in the generation of significant amounts of C3a. Assessment of the C3a levels by the modified RIA in serial plasma samples from patients who underwent cardiopulmonary bypass, yielded results very similar to those described in the literature for the established C3a-RIA. Thus, the modified C3a-RIA offers a convenient alternative for the detection of C3a in plasma samples.