Levamisole is a heterocyclic compound that is an effective antihelminthic agent and is also immunoregulatory. Its likely immuno-regulatory mode of action is by mimicry of the thymic hormone thymopoietin. Chemically, levamisole may form a thymopoietin-mimetic tertiary structure, stimulate lymphocytes by its imidazole component, or be metabolized to O.M.P.I., a reducing compound which affects radical scavenging in multiplying lymphocytes. Physiologically, thymopoietin affects many components of the immune system including both neutrophils, macrophages, and lymphocytes and its therapeutically important actions are probably targeted at stimulation of phagocytosis and stimulation of regulatory T cells to restore homeostasis in a perturbed immune system.