Levamisole--(L)--1-2, 3, 5, 6--tetrahydro-6- phenylimidazo (2, 1-b)-- thiazol monohydrochloride, a simple chemical, first introduced as a broad spectrum antihelmintic , has been recently on the list of the nonspecific active immunostimulants together with BCG, Corynebacterium parvum, polyribonucleosides and transfer factor (TF). This, however, is oversimplification and may cause some confusion because levamisole, in contrast to the so-called immunostimulants, does not stimulate immunity above the normal level in man or prevent the primary growth of most experimental tumors in immunologically normal animals. Levamisole acts as an antianergic chemotherapeutic agent as it restores cell-mediated immunity in immunodepressed patients and prolongs the remission period. It even increases the number of long-term survivors when used as an adjunct to cytoreductive therapy in several animal cancer models. Levamisole is rapidly absorbed from the gastrointestinal tract and from injection site and is very well distributed in all tissues. Levamisole increases phagocytosis by polymorphonuclear cells or macrophages when added to these cells or given to donor animals and humans. The effect was pronounced on hypofunctional cells from patients and it was weak or absent on cells from normal donors. Chemotactic responsiveness of polymorphonuclear cells and monocytes from patients with defective leucocyte motility could be enhanced by levamisole added in vitro or given in vivo. The leucocyte migration inhibition in response to antigenic stimulation could be restored when levamisole was administered to anergic patients or added to their cells in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)