The stability of prothrombin and factor VII was studied using accelerated degradation tests in three preparations of freeze-dried pooled normal plasmas. In a previous report (Brozović, Gurd, Robertson, and Bangham, 1971) factor X was shown to be relatively unstable in these preparations of freeze-dried plasma: it was calculated that up to 8% of the original factor X activity would be lost after 10 years at -20 degrees C, up to 54% at 4 degrees C, and up to 90% at room temperature. The losses of factor VII activity were estimated to be negligible at -20 degrees C, between 2 and 18% at 4 degrees C, and between 20 and 70% of the original activity at 20 degrees C, after 10 years of storage. Prothrombin was found to be less stable than factor VII: the expected loss in 10 years at -20 degrees C may be up to 4%, at 4 degrees C up to 30%, and at 20 degrees C up to 83% of the initial activity. These findings indicate that in freeze-dried plasma prothrombin as well as factor X may be insufficiently stable for plasma to serve as long-term reference material for the standardization of the one-stage prothrombin time. Moreover, the loss of prothrombin and factor X in freeze-dried plasma stored at 4 degrees C may be so high that when it is required to preserve these factors it may be necessary to store freeze-dried plasma at lower temperatures.