This unit contains protocols that can be used to measure mannan-binding lectin (MBL) levels and MBL pathway activity in human plasma or serum. Using a modification of the conventional ELISA, the detection reagent used in the methods described (e.g., an antibody or streptavidin) is labeled with Eu3+ instead of an enzyme. The sensitizing reagent used in these assays employ the use of anti-MBL antibody or mannan. The unit also includes an MBL/MASP complex activity assay to evaluate MBL activity in serum or plasma samples, which allows for the measurement of the specific activity of the MBL pathway.