The complement system is an important component of innate immunity. Complement-derived products mediate functions contributing to pathogen killing and elimination. However, inappropriate activation of the system contributes to the pathogenesis of immunological and inflammatory diseases. Complement component 3 (C3) occupies a central position because of the manifold biological activities of its activation fragments, including the major fragment, C3b, which anchors the assembly of convertases effecting C3 and C5 activation. C3 is converted to C3b by proteolysis of its anaphylatoxin domain, by either of two C3 convertases. This activates a stable thioester bond, leading to the covalent attachment of C3b to cell-surface or protein-surface hydroxyl groups through transesterification. C3b is further cleaved into iC3b, C3c, C3dg and C3f. C3b and iC3b function as opsonins, they act through different complement receptors, complement receptor 1 (CD35) and complement receptor 3 (CD11b/CD18), respectively.The mouse C3b ELISA assay is specific for the cleaved C3 fragments C3b, iC3b, and C3c, and for activated C3. Therefore, positive reactivity in plasma or serum is associated with activation of the complement cascade and C3 cleavage. In case of an acute inflammatory reaction, lots of C3 are processed into the products recognized by the assay, making this assay a useful tool for measuring the acute inflammatory response. The assay is less useful for assessing chronic inflammatory conditions since minimal reactivity may be observed. In such cases, primarily the C3dg product resides at the place of inflammation (C3c being cleared) which is not recognized by the assay. The chronic processing/activation of C3 is taking place at a lower level, which would reduce detection of the C3 fragments C3b, iC3b, and C3c. Beware that complement activity levels are mouse strain dependent and might be affected by the way the samples are collected and processed. C3 levels can differ between healthy and diseased animals, the optimal dilution can be different between these statuses.