A monoclonal antibody directed against the immunosuppressive ligand programmed cell death-1 ligand 1 (PD-L1; cluster of differentiation 274; CD274), with potential immune checkpoint inhibitory and antineoplastic activities. Upon administration, anti-PD-L1 monoclonal antibody RC98 specifically targets and binds to PD-L1, blocking its binding to and activation of its receptor programmed cell death 1 (PD-1; cluster of differentiation 279; CD279). This reverses T-cell inactivation caused by PD-1/PD-L1 signaling and enhances the cytotoxic T-lymphocyte (CTL)-mediated anti-tumor immune response against PD-L1-expressing tumor cells. PD-L1 is overexpressed by many human cancer cell types. PD-L1 binding to PD-1 on T-cells suppresses the immune system and results in immune evasion. PD-1, a transmembrane protein belonging to the immunoglobulin superfamily (IgSF) expressed on activated T-cells, is a negative regulator of the immune system that limits the expansion and survival of CD8-positive T-cells.