The aim of the study was to examine the morphology of the bone marrow of mice after stimulation with cyclophosphamide (Cy). The experimental mice were given a single intraperitoneal injection with 250 mg/kg bw cyclophosphamide. After 2, 4 and 6 days of experiment the femurs were obtained for morphological study. On the 2nd day after the mobilisation of the mice with Cy destruction of the bone marrow was observed with a decrease in the haematopoietic compartment and an increase in the area occupied by sinusoids filled with erythrocytes. Erythrocytes were located among the haematopoietic cells, which indicated that the endothelial barrier had been disrupted. On the 4th day after treating the mice with Cy, repair processes in the bone marrow were conducted, including macrophages. The cells filled with haemosiderin migrated from the extravascular compartment of the bone marrow into the lumen of the sinusoids. There were proliferating cells among the haematopoietic cells. On the 6th day the morphology of the bone marrow was similar to the morphology of that in the control mice. However, more haematopoietic cells were visible compared to the control bone marrow. The presence of an increased number of leucocytes in the sinusoid lumen in comparison with the control suggested that at that time the migration of haematopoietic cells from the bone marrow had been initiated.