Saline, Triton X-100 and acid-soluble inorganic phosphate were quantitatively assayed from freeze-dried dentine of sound and carious human teeth. The phosphate levels were measured from 5.0-mg (dry weight) samples of drilled dentine powder taken from 3 to 6 areas of a single tooth (93 teeth were used). The highest concentrations of total phosphate were in the peripulpal area of the crown. Root dentine contained less total phosphate than did crown dentine. The amount of total phosphate increased significantly with the age of the teeth. A decrease of 30–60% was observed when the dentine became carious. Hard dentine in the vicinity of the caries lesion (sclerotic, pigmented dentine) indicated about the same phosphate content as the dentine of corresponding areas of intact teeth. The mean NaCl-soluble fraction of the hard dentine was relatively constant. The values in crown odontoblastic dentine, however, revealed significant changes as a function of carious attack. The NaCl-soluble phosphate of soft caries dentine was somewhat higher than that of hard dentine. The possible role of NaCl-soluble phosphate ions in the regulation of phosphatase activity in the dentine is discusse