Saline, as well as detergent-soluble, phosphomonoesterase activity was quantitatively assayed from freeze-dried dentine in sound and carious human teeth. The activity was measured from 10-mg samples of dentine taken from 3 to 6 different areas of one tooth (76 teeth). The rate of hydrolysis of p-nitrophenylphosphate (p-NPP) was measured and calculated per milligram protein or per milligram of the dry weight of the sample both at acidic (possible reflecting acid phosphatase activity EC 3.1.3.2) and alkaline (alkaline phosphatase EC 3.1.3.1) pH values. At the acidic level, Triton X-100 treatment revealed higher activity than did saline treatment in sound dentine. In dentine from carious teeth p-NPP was hydrolyzed most rapidly by the enzymes solubilized in saline solution. The highest activity was observed in NaCl solution solubilized soft carious dentine from gangrenous teeth. At alkaline pH values Triton X-100 revealed higher activity, both in sound and carious dentine, when compared to saline treatment. The most rapid rate of hydrolysis was observed in the peripulpal dentine layer under incipient carious changes in dentine. The increase of this activity was also reflected in the peripulpal area of the apex. These observations support the hypothesis that the dentine as a tissue reacts already in the early stages of caries both anabolically and catabolically.