C3a levels in plasma are usually measured by a competitive inhibition radioimmunoassay (RIA) using 125I-labelled C3a-desArg and antibodies to C3a capable of detecting C3a determinants which are also present on the native C3. Therefore, prior to the assay native, non-cleaved C3 has to be removed completely from the C3a-containing sample by precipitation. We developed a new rapid two-site sandwich ELISA system for the quantitation of C3a-desArg in plasma. This immunoassay uses a monoclonal antibody (mAb H466) reacting with C3a-desArg but not with C3. The reactivity of mAb H466 with a neoantigenic determinant of C3a-desArg permitted the direct quantitation of C3a-desArg without removal of C3 from the sample. The mAb H466 was used as a capture antibody and bound C3a-desArg was detected with a second peroxidase-labelled anti-C3a mAb. The lower limit of detection of C3a-desArg in this ELISA was 1 ng/ml. The C3a-desArg levels measured in the plasma samples of various patients were found to differ over a wide range. A good correlation was observed between the results obtained in the RIA and those obtained in the ELISA (r = 0.95). High levels of C3a-desArg were detected in plasma from patients with multiple trauma and patients undergoing haemodialysis. The C3a-desArg assay described should facilitate the routine quantitation of C3a in samples of plasma.