Failure of host defense systems associated with malignancies may be attributable to the tumor, to cytoreductive therapy or to combined endogenous and iatrogenic influences. Management of the resulting increased susceptibility to infections may require supplementation of antibiotic therapy with additional forms of treatment, including passive immunization with antibodies. This review discusses the use of immunoglobulin preparations for intravenous administration (IVIG) in patients with secondary immunodeficiencies associated with neoplasia. A suitable model for evaluating the prophylactic effect of IVIG is chronic lymphocytic leukemia. Many observations suggest that IVIG reduces the frequency of acute respiratory infections. Another malignant condition with decreased serum levels of polyclonal immunoglobulins and high frequency of infections is multiple myeloma. A crossover study recently demonstrated that IVIG significantly (P less than 0.01) reduced the frequency of respiratory tract infections in these patients. Furthermore the prophylactic effect of IVIG was evaluated in patients with small cell carcinoma of the lung. In a randomized prospective trial it was noticed that IVIG applied during intensive chemotherapy and irradiation courses significantly (P = 0.04) reduced the frequency of infections. Evidence for a therapeutic effect of IVIG was obtained in adult tumor patients and in children with leukemia or non-Hodgkin's lymphoma who developed severe varicella-zoster virus infections. The treatment effectively controlled fever, skin lesions and neuralgia and prevented progression of the infection. Therapeutic usefulness of IVIG in bacterial infections is still based on anecdotal evidence. Experimental data suggest that in addition to effects mediated by specific antibodies, nonspecific interactions of IgG molecules with Fc-receptors on macrophages may be clinically important.