A sensitive double antibody enzyme-linked immunosorbent assay (ELISA) for quantification of C3 activation products in human plasma, synovial fluid, and cerebrospinal fluid is described. The monoclonal antibody MoAb bH6, which is specific for a C3 neoepitope expressed on C3b, iC3b, and C3c, was used as capture antibody. Detection antibody was a polyclonal rabbit anti-human C3c followed by development with a peroxidase-conjugated anti-rabbit Ig antiserum. The activity in normal human EDTA plasma was 1.5% of that in zymosan-activated serum (ZAS). The interassay and intra-assay coefficients of variation were 15% and 3%, respectively. The lower detection limit was 0.0005% of the ZAS standard. Reference range (1.1-2.1% of ZAS) was obtained by measuring EDTA plasma from 40 healthy blood donors. A positive correlation rs = +0.92; P less than 0.0002) was found between the present assay and an already established C3'g' activation ELISA, when samples from 16 patients were examined in both assays simultaneously. The present assay and an assay detecting the terminal complement complex showed virtually identical activation patterns in consecutively drawn samples from a patient undergoing extracorporal circulation.