The authors have compared the values of total and bone serum alkaline phosphatases in an osteoporotic population and a control group. The total activity of alkaline phosphatases was determined by a kinetic method at 30 degrees C and alkaline phosphatase isoenzymes were separated by agarose gel electrophoresis in presence or absence of wheat germ lectin. The authors have confirmed reported data concerning the physiological variations of alkaline phosphatases in the control group; they have therefore divided each group in several sub-populations according to age and sex in order to obtain accurate comparisons. Mean values of total and bone alkaline phosphatases were greater in the osteoporotic population than in the control group regardless of age or sex. Nevertheless, significant differences were obtained only with 50 to 75 year-old subjects (men or women) for total alkaline phosphatases and with 50 to 75 year-old women for bone alkaline phosphatase.