To the Editor:— From Dr. Sise's reply to my letter concerning the interpretation of prothrombin-time methods ( JAMA 191 :604 [Feb 15] 1965), it is clear that the crux of the misunderstanding is in the use of the term true prothrombin . If Dr. Sise will measure the prothrombin in newborn blood with his modification of the one-stage test which he claims measures true prothrombin, he will find, as was originally established with the unmodified prothrombin time, that the prothrombin is approximately the same as in adult blood. With the Iowa twostage method, the prothrombin concentration is found to be only about 25% of the adult level, or in other words, the latter contains four times more prothrombin. One could hardly conclude that the two-stage method measures false prothrombin. I have offered the explanation that the two-stage method estimates total prothrombin, whereas the onestage determines only free prothrombin. In adult blood 75%