Immunological reactivity and factors of nonspecific body defense were studied in 35 patients with advanced chronic myeloid leukemia (CML). Prior to cytostatic therapy, CML patients have secondary combined immunodeficiency manifesting by inhibition of T-system immunity and phagocytic neutrophil activity. Repeated courses of myelosan chemotherapy aggravate the above immune defects. The adjuvant immunomodulator tactivin corrects cellular immunity and reduces the occurrence of infectious complications 1.77-fold.