The CD40 Bioassay is a bioluminescent cell-based assay for measuring the potency and stability of ligands or agonist antibodies and other biologics that can bind and activate CD40. The cell surface molecule CD40 expressed by B cells, dendritic cells and monocytes is a member of the tumor necrosis factor receptor superfamily. CD40 ligand (CD154) is the primary ligand for CD40 and is expressed by activated T cells, which are critical regulators of cellular and humoral immunity. Signaling via CD40 triggers activation of antigen presenting cells (APC). Agonist CD40 antibodies were found to mimic the signal of CD40 ligand and were capable of substituting for the function of CD4+ helper T cells in murine models of T-cell-mediated immunity. Therefore, agonist CD40 antibodies can rescue the function of APC in tumor-bearing hosts and restore effective immune responses against tumor antigens. Subsequent data from multiple preclinical models has demonstrated synergistic enhancement from combining CD40 agonists with cytotoxics, especially chemotherapy. The CD40 Bioassay reflects the mechanism of action (MOA) of biologics designed to activate the CD40 receptor. Specifically, CD40-mediated luminescence is detected following the addition of CD40 agonist antibodies or CD40 ligand. The bioassay is prequalified according to ICH guidelines and shows the precision, accuracy and linearity required for routine use in potency and stability studies.