1. Friend, C., J. exp. Med., 105, 307–318 (1956).
2. Rauscher, F. J., J. natn. Cancer Inst., 29, 515–543 (1962).
3. Kirsten, W. H., Mayer, L. A., Wollmann, R. L., and Pierce, M. I., J. natn. Cancer Inst., 38, 117–139 (1967).
4. Kirsten, W. H., and Mayer, L. A., J. natn. Cancer Inst., 39, 311–335 (1967).
5. Harvey, J. J., Nature, 204, 1104–1105 (1964).