Levamisole, a drug initially used for its antihelminthic properties has been recently emphasized when Renoux proved its immunostimulating effects in mice en 1971. Since this date, numerous publications concerned the immunological activity of this drug in animals and men. These come to the conclusion that levamisole is capable of restoring cellular immunity as demonstrated by clinical and biological tests (restoration of delayed skin hypersensitivity, increase of the percentage of rosette forming cells and PHA transformed cells). In men, its seems possible to confirm the therapeutic activity of this molecule in a wide range of disease with suspected or proven immunological deficiency. In different forms of cancer, when most of the tumor volume is first reduced, levamisole therapy significantly increases both remission and survival. In auto immune diseases (rheumatoid arthritis, systemic lupus erythematosus), levamisole seems to be strikingly effective. In viral infections (zoster, recurrent herpes, aphthous stomatitis) levamisole is able to reduce the duration of outbreaks and prolong the disease free interval. Finally, encouraging results have been obtained in patients with lepromatous leprosis. Other conditions, where an immunological deficiency is suspected are under study with levamisole therapy. This low molecular weight synthetic drug is perhaps the first pharmacological agent which acts on the host defense mechanisms.