The effect of the extract of the Bursa of Fabricius of broiler chickens on the possibility of correcting the immunodeficiency state (ID) of C57BL/6 and C57BL/10 mice caused by a single intraperitoneal injection of cyclophosphamide (CFA) at a dose of 200 mg/kg was studied. Bursa extract was injected daily intraperitoneally for 7 days at a dose of 0.1 mL/20 g. The formation of ID and its correction were assessed by hematological parameters and myelogram counting. Administration of CFA to mice of both lines had cytostatic effects on lymphoid and erythroid bone marrow lineages and enhanced neutrophil cell proliferation, as well as blood anemia, lymphopenia, and granulocytosis. Injections of bursa extract after CFA administration were accompanied by proliferation of lymphocyte, neutrophil, and erythrocyte precursors in bone marrow in mice of both lines, which contributed to an increase in total white blood cell count and anemia correction.