Biologically important apatite analogues have been examined by Fourier Transform Infrared Spectroscopy (FT-IR), and a method developed to quantitatively assess their crystalinity. Changes in the phosphate v₁ and v₃ regions, 900–1,200 cm^-1, for a series of synthetic (containing hydroxide, fluoride, or carbonate ion) and biological apatites with crystal sizes of 100–200 Å were analyzed with curve-fitting and second derivative spectroscopy. The v₁,v₃ contour was composed of three main subbands. Correlations were noted between two spectral parameters and crystal size as determined by x-ray diffraction. The percentage area of a component near 1,060 cm^-1 decreased as the length of the c-axis of the hydroxyapatite (HA) compounds increased, while the frequency of a band near 1,020 cm^-1 increased with increasing length of the apatite c-axis. These parameters are thus proposed as indices of crystallinity for biological (poorly crystalline) HA. The FT-IR spectra of highly crystalline apatitic compounds were also analyzed. For crystal sizes of 200–450 A, the percentage area of the phosphate v₁ band (near 960 cm^-1) decreased with increasing HA crystal size. IR indices of crystallinity have thus been developed for both well crystallized and poorly crystallized HA derivatives. The molecular origins of the various contributions to the v₁,v₃ contour are discussed, and a preliminary application of the method to a microscopic biological sample (rat epiphyseal growth plate) is illustrated.