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Three instances of malignant teratomas with positive reaction for alpha‐fetoprotein (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. Two of these tumors originated in the testis and a third in the presacral region. The microscopic sections of these malignant teratomas were carefully reviewed and it was concluded that they are properly classified as yolk sac tumor or endodermal sinus tumor. The 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 at early embryogenesis. This phenomenon will conversely lend an additional support to the morphologic contention that these tumors are of yolk sac origin and deserves further investigation.

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