Summary When a beam of pulsed sound is passed between two transducers a fixed distance apart, the transit time depends upon the velocity of sound in the medium separating them. Because the velocities of sound in soft tissues are similar and much less than in bone, an instrument which records changes in transmission time can be used to determine the mass of bone present in an intact limb. When measurements were made on samples of cortical bone and of the humerus of a rabbit (either intact or stripped of the soft tissues) there was good agreement between the predicted and subsequently chemically determined mass of calcium present.