The paper provides the results of examining about 3,000 industrial workers exposed to various xenobiotics. Various phases of immune responses were noted in 40-60% of persons with nonspecific chronic pulmonary, gastrointestinal, gynecological diseases at remission. 10-20% of the industrial workers under study were diagnosed as having secondary immunodeficiency whose criteria were the presence of one of clinical syndromes, a sharply significant reduction in immunoglobulins, normal levels of circulating immune complexes and normal values of the lymphocytic enzymes alpha-glycerophosphate dehydrogenase, succinate dehydrogenase, lactate dehydrogenase. The stimulation of the immune system in clinically healthy individuals who have a service length of 3 year or more should be regarded as development of an immune response to a xenobiotic or its metabolites.