The total number of nucleated cells in the long bone of a mouse can be determined with some accuracy. Thus, in this species, values for marrow cells can be expressed as a total cell count per bone, a more meaningful number than values expressed as concentration as is done in most studies of other species. If the percent of total marrow in the skeleton that is contained in that bone is known, values can be expressed as “per mouse” (total marrow mass)–‐a still more meaningful value than values per bone. The total marrow mass of mice has been calculated previously on the basis of nucleated cells per humerus or per femur and the percent of the total marrow contained in that bone. However, that percent was based on rather tedious dissection of the entire skeleton and determining the amount of ⁵⁹Fe that had been taken up by each bone. In the present study, mice were injected with ⁵⁹Fe, skinned, and eviscerated. The carcass was then either cooked and all bones dissected out or simply cut into pieces containing various bones or bone groups. The percent of ⁵⁹Fe taken up by various bones or bone groups as measured by the two techniques was virtually identical. The percent distribution between various bones was found to be fairly constant between 4 and 18 h after ⁵⁹Fe injection and the same in mice aged 3 or 12 months. This simplified technique makes the measurement of total marrow mass a practical addition in studies of murine hematopoiesis.