T h r e e instances of malignant teratomas with positive reaction for alphafetoprotein (AFP) are presented. Sequential evaluations in them revealed that the production rate of AFP closely paralleled the amount of the viable malignant tumor tissue which fluctuated in the course of the diseases. T w o of these tumors originated i n the testis and a third in the presacral region. The microscopic sections of these malignant teratomas were carefully reviewed and i t was concluded that they are properly classified as yolk sac tumor or endodermal sinus tumor. T h e occurrence of AFP in these tumors is best explained by the yolk sac notion, because it is known that large amounts of AFP are synthesized not only by the fetal liver, but also by the yolk sac a t early embryogenesis. This phenomenon will conversely lend a n additional support to the morphologic contention that these tumors are of yolk sac origin and deserves further investigation.
LPHA-FETOPROTEIN (AFP), WHICH IS F O U N D
A only in the fetal serum and disappears from the human serum shortly after birth, reappears in pathologic states, such as hepatocellular carcinomas, teratocarcinomas, and some hepatic disorders. Synthesis of AFP by the fetal liver, by hepatocellular carcinomas, and by regenerating hepatic cells has been