Serum alkaline phosphatase (AP) activity is elevated in hepatobiliary disease, bone disease, pregnancy and certain neoplasms. Recently we experienced marked elevation of serum AP activity after administration of albumin preparation in nephrotic patients who suffered from hypovolemic symptoms. So serum AP activity and the isoenzymes in the albumin preparations & patient's serum after the administration of albumin preparation were studied. Serum AP activity was significantly higher after administration of albumin preparation (318+/-101 IU/L) then before (123+/-43 IU/L). The predominant isoenzyme after administration of albumin was placental type, while liver and bone type was predominant before administration. AP activity in albumin preparation was high (2,133+/-1,410 IU/L) and the isoenzyme was mostly placental type. So we concluded that marked elevation of serum AP activity after administration of albumin was traced to the placental type AP isoenzyme in some albumin preparations which was manufactured from the plasma of placental origin. Elevated serum AP activity like these may lead to erroneous interpretation. Manufactures should notify alkaline phosphatase activity in albumin preparations of placental origin.