The immunoregulatory activity of aporphinoid alkaloid oxoglaucine was studied in delayed-type hypersensitivity (DTH) reaction and Klebsiella pneumoniae infection in normal and immunosuppressed mice. The alkaloid enhanced DTH reaction when applied during the developing phase and abolished the inhibitory action of cyclophosphamide. Oxoglaucine possessed a restorative effect in K. pneumoniae infection in immunosuppressed mice in combination with indomethacin, prednisolone or artemisinin, as evaluated by the number of survivors and mean survival time. These data suggest that oxoglaucine affects selected lymphocyte clones responsible for antiinfectious host resistance. Its use as a second agent in combination with another immunosuppressant might enable reduction in the dosage or time of application.