1. A noninvasive in vivo assay of the mineral content of a bone in an animal comprises: (a) placing the part of the body of the animal containing said bone to be analyzed within the operational range of a receiver coil of a NMR spectrometer; (b) recording a .sup.31 P NMR spectrum of the bone while keeping the bone stationary; and (c) then comparing the quantitative spectral results obtained in step (b) to the spectral results obtained with a reference standard containing a known concentration and quantity of a reference compound which responds to .sup.31 P NMR spectroscopy in a manner similar to that of bone mineral but which does not overlap the resonance of bone material.