The densities of about 1000 limb bones, excised from 110 carcasses of four breeds of cattle ranging in age from 6 to 36 months, were measured by the method of weighing in water. The density of individual bones varied considerably ranging from 1·24 to 1·71 gcm-3 Some of this variation was due to differences between the various bone types within a limb, the cannon bones being more dense than the tibia and radius/ulna which in turn were more dense than either the humerus or femur. Age and breed also influenced density; the older the animal the more dense were its bones and the bones of Jersey cattle were more dense than those of Herefords of the same age. These findings conflict with an assumption that has been made hither to in theoretical analyses of the densitometric method of estimating the composition of carcasses, where skeletal density has been generally taken as constant.