A method is described for quantitative measurement of C3d in plasma and synovial fluid by the use of rocket immunoelectrophoresis after fractionation of the samples with 22% polyethylene glycol. This method has the advantage over radial immunodiffusion of being more sensitive (detecting C3d down to 3 mg/l) whilst proving equally reproducible. Investigations indicate that the collection of blood samples in EDTA prevents in vitro activation of C3 even after storage for up to 6 h at room temperature and up to 12 weeks at -70 degrees C. Elevated levels of C3d were found in a proportion of SLE and RA plasma samples and in synovial fluids from patients with inflammatory synovitis. It is suggested that C3 conversion in vivo may be assessed by measurement of C3d by the technique described, and when used in conjunction with measurements of complement components and immune complexes, offers a means of investigating complement catabolism by the classical and alternative pathways.