1. A.B. Otnaess and I. Orstavik, Infect. Immun. 33, 459, (1981).
2. E. Weinberg, Science 184, 952 (1974).
3. J.J. Bullen, H.J. Rogers and E. Griffiths, Curr. Top. Microbiol. Immun., 80, 1, (1978).
4. J.M. Dolby and P. Hoover, J. Hyg. (Camb). 83, 255, (1979).
5. H.J. Rogers, Immunology, 30, 425, (1976).