The first part of this paper focuses on the secondary immunodeficiencies caused by immunosuppressive drugs and environmental chemicals. The mechanism of action and the therapeutic effects of azathioprine, glucocorticosteroids, cyclophosphamide, cyclosporin A, and anticancer drugs will be discussed. The immunotoxic actions of benzene, polychlorinated and polybrominated biphenyls, tetrachlorodibenzo-paradioxin, polycyclic aromatic hydrocarbons, various insecticides, metals and abused drugs or drugs with side effects are also described. Immunorestorative approaches are discussed in the second part, based on our current knowledge of the immune system and the lymphokines and hormones involved in its function. The usefulness of thymic hormones, interleukin 2, macrophage growth factor/colony stimulating factor, and interferons as immunotherapeutic agents is considered. In addition, we address chemically defined drugs, such as levamisole, diethyldithiocarbamate, isoprinosine, muramyl dipeptides, azimexon, ciamexon, bestatin, tuftsin, and pyrimidinoles, and finally conclude that combined use of different immunostimulants may be the most effective way to treat the above mentioned immunodeficiencies.