OK-432 is an immunomodulatory reagent prepared from the lyophilized, penicillin-inactivated Su strain of Group A Streptococcus pyogenes. In peripheral blood mononuclear cell (PBMC) cultures, OK-432 activates myeloid cells (monocytes/macrophages and dendritic cells), which subsequently release IL-12 and other cytokines that drive natural killer (NK) cell activation. Activated cells exhibit increased IFN-γ production, degranulation, and cytotoxic activity. OK-432 can also enhance neutrophil-mediated tumoricidal effects (e.g., via CD11b/CD18–ICAM-1 interactions) and induce LAK-like activity in lymphocytes. Owing to these immunostimulatory properties, OK-432 is widely used in vitro as a research-use-only NK-cell activator for NK/LAK activation workflows, ADCC assay development, dendritic-cell maturation, and tumor-immunology studies.