Bupleurum-containing compounds, such as KY88 Liver Livo are thought to have immunomodulatory effects. This study investigated the effects of KY88 Liver Livo on the mitogenic induction of lymphocytes in vitro. Fifteen healthy human adult volunteers, aged between 20 and 50 years, provided peripheral blood samples from which lymphocytes were obtained by Ficoll-Hypaque centrifugation. The separated lymphocytes were stimulated by phytohaemagglutinin and KY88 Liver Livo in varying concentrations for 72 hours, with greater cluster and colony formation evident compared with lymphocytes in a control preparation. KY88 Liver Livo was also found to induce the secretion of granulocyte-macrophage colony-stimulating factor in a dose-dependent fashion. These preliminary in vitro studies suggest that KY88 Liver Livo may have potential clinical value in the treatment of chronic viral infection and in the management of immunocompromised patients.