Experimental ectopic calcification caused by intravenous injection of lead acetate (Pb−Ac) followed by subcutaneous injection of polymyxin B sulfate (PMX) in the rat was studied by the methods of quantitative chemical analysis and X-ray diffraction analysis using samples freed of organic matter by low temperature ashing (LTA). In all specimens, X-ray diffraction data showed the presence of an apatitic phase. In addition, several unknown peaks, the intensities of which weakened with time, were found. These peaks were established to be those of lead pyrophosphate (Pb2P2O7) when compared with the Joint Committee on Powder Diffraction Standards (JCPDS), formerly the ASTM index. Improvements in crystallinity of the apatitic phase was accompanied by the increases in the amounts of calcium, phosphorus and carbonate in the LTA ash. The amount of magnesium, on the contrary, decreased from 3 to days. The molar Ca/P ratio was near to 1.5 up to 10 days and then increased to 1.59 at 40 days. When the LTA ash was heated at 600° C, the major crystal phase wasβ-tricalcium phosphate and the minor phase was hydroxyapatite up to 10 days; after that the relationship between the two phases was reversed. It is suggested that the characteristics of calcium phosphate in the native state was transformed between 10 and 20 days after administration of Pb−Ac and PMX.