Age variation of mouse serum alkaline phosphatase activity was ascertained with 263 CBAxC57BL/6 F1, hybrid mice 29 to 366 days old, by the Kind and King method. The values were 10.0 to 22.0, 5.2 to 7.9 and 4.0 to 5.1 K.A. units at 30, 80 to 200 and over 200 days of age, respectively. The mean values and their variation ranges gradually declined with age. The activity in females was constantly higher than in males.Serum alkaline phosphatase activity of 107 Gardner's osteosarcoma‐bearing mice was determined. At 10 to 15 days after transplantation when the grafted tumors were smaller than 3times3 mm, the values were all less than 10.0 K.A. units of those of normal mice. After the tumors had grown to over 3times3 mm in size, the values rose to the extent of 500.0 K.A. units with the increase of tumor size. The increased serum alkaline phosphatase activity was regarded as a marker of tumor growt