We separated isoenzymes of alkaline phosphatase (ALP; EC 3.1.3.1) in 1383 sera of normal individuals (ages 4-65 years) by agarose electrophoresis with the Isopal system (Analis). As expected, the predominant isoenzyme in children was of bone origin, and almost all (99%) of the children had low activities of a second bone fraction, "bone variant" ALP. The "bone variant" disappeared after age 17 in girls and after age 20 in boys. The highest (median) bone ALP activity was reached at age 9 to 10 in girls and at age 13 to 14 in boys, followed by a gradual decline in girls and a steep decline in boys. During adulthood, activity of the bone fraction was constant and no significant differences were observed between sexes, neither for bone nor for liver ALP activity. The latter remained almost unchanged throughout life. We observed no high-Mr ALP activity in children, whereas sera from 60% of the adults contained low activities of high-Mr ALP. Intestinal ALP (soluble form) and "intestinal variant" ALP (hydrophobic form) were frequently present, in 21% and 37% of all samples, respectively. No significant differences were observed between age groups and sexes for the intestinal isoenzymes.