The data on the content of nucleated bone marrow cells in 17 skeletal portions of the hybrid F1 (CBA X C57B1) laboratory mice weighing 18-21 g are presented. The bone marrow content in the bones of the spine, head, inferior limb, pelvis, upper limb, sternum and ribs constituted 33.7; 19.6; 17.6; 11.9; 8.2 and 9.0% of the total amount, respectively. The epxerimental results are compared with the literature data.