PURPOSE: To provide an agent for inducing production of nitrogen oxide, capable of inducing the production of nitrogen oxide in living body or cultured cell and effective as an antitumor agent or a therapeutic agent for infectious diseases.

CONSTITUTION: The objective agent for inducing the production of nitrogen oxide contains human interleukin 6 (human IL-6) as an active component and preferably further about 10-100 pts.wt. (based on 100 pts.wt. of IL-6) of a lipopolysaccharide(LPS) as another active component. The progression of the symptom of tumor can be suppressed by the local administration of the agent to a tumor to which a nitrogen oxide is expected to be effective and, further, the agent is effective for the treatment of various infectious diseases by the stimulation of the phagocytotic action of macrophage. Nitrogen oxides having various applications and represented by NO, NO2-, NO3- and nitrosothioproline having pharmacological action as a vascular smooth muscle relaxation factor can be produced by cluturing macrophage in the presence of IL-6 or IL-6 and LPS.