Meaningful quantitation of complement activation is difficult by conventional techniques, because of the dynamic equilibrium status of complement components. Assays for complement-derived split products have been useful adjuncts, but until recently have been cumbersome and only semiquantitative. The recent introduction of superior assay methods, especially immunoassays for the complement derived anaphylatoxins, has enhanced such study by allowing more sensitivity, specificity and reproducibility. such assays are already being applied to the study of diseases in which complement activation may play a role, as well as to the evaluation of biocompatibility of various prosthetic materials and drugs. 'Bedside' clinical utility of the tests is still limited, but may grow rapidly if assays are devised which are capable of more rapid and less expensive performance.