This article examines the effect of cyclophosphamide (CPA) on hematological parameters in C57BL/6 mice and the possibility of correcting these changes with bursa of Fabricius extract. Administration of CPA leads to the development of an immunodeficiency state characterized by severe lymphopenia, anaemia, and changes in the leukocyte population. A decrease in the number of red blood cells, haemoglobin, and haematocrit is observed, as well as changes in the parameters of red blood cells that indicate a disorder of haemoglobin synthesis. At the same time, there is an increase in the proportion of monocytes, eosinophils, and basophils. The administration of bursa of Fabricius extract has an effect on the parameters of white blood cell and increases the total number of white blood cells, as well as the proportion of middle blood cells, and granulocytes. The red blood also shows positive dynamic, but the parameters do not reach the control values. The MPV and PDV indices of platelets do not change. The results obtained show that the extract from the bursa of Fabricius promotes the restoration of haematological parameters affected by the administration of CFA and indicate the prospects for further investigation of its immunomodulatory properties.