Quantitative external counting methods for the in vivo determination of the activity uniformly distributed in an area of interest (lesion) have been developed. The theoretical work has been extended to include the effects of regions of differing attenuation coefficient and of non-target organ activity in the tissue surrounding the lesion. A clinical quantitative procedure utilizing a "calibrated" gamma camera and associated computer system is described. Studies including a bone substitute material in a water phantom confirmed the basic theoretical work.