Serum alkaline phosphatase (A.P.) activity was assayed in 859 healthy children aged 6-18 years on the island of Euboea, Greece. Normal A.P. values (less than 100 U/l) were found in 673 (78.6%), borderline values (100-120 U/l) in 137 (15.7%) and increased values (greater than 120 U/l) in 49 children (5.7%). In both sexes the highest A.P. levels were observed at the age of 13 years followed by a gradual reduction to adult values at the age of 17 years. In general, A.P. levels were higher in boys (mean 64.4 +/- 23.3 U/l) than in girls (mean 56.3 +/- 21.6 U/l). After summer holidays the A.P. was reinvestigated in 36 children who had had high winter values. The levels had then considerably decreased (p less than 0.001); in all but three cases, A.P. levels had returned to normal. Fifteen children with normal A.P. activity at the time of the first examination were reinvestigated after summer holidays and showed also a decrease in A.P. activity.