1. Van Duuren, B. L. & Orris, L. Cancer Res. 25, 1871–1875 (1965).
2. Estensen, R.D. et al. in Control of Proliferation in Animal Cells (eds Clarkson, B. & Baserga, R.) 627–634 (Cold Spring Harbor Laboratory, New York, 1973).
3. Abb, J., Bayliss, G. J. & Deinherdt, F. J. Immun. 122, 1639–1642 (1979).
4. Mastro, A. M. & Muella, C. G. Expl Cell Res. 88, 40–46 (1974).
5. Friend, C. W., Scher, W., Holland, J. G. & Sato, T. Proc. natn. Acad. Sci. U.S.A. 68, 378–382 (1971).