1. Hellmann, K., and Hawkins, R. I., Nature, 201, 1008 (1964).
2. Hawkins, R. I., and Hellmann, K. (in preparation).
3. Biggs, R., and Macfarlane, R. G., Human Blood Coagulation and its Disorders, third ed. (Blackwell Scientific Publications, Oxford, 1962).
4. Margolis, J., J. Clin. Path., 11, 406 (1958).
5. Seegers, W. H., Prothrombin (Harvard Univ. Press, 1962).