342 B. NAKIC, A. KASTELAN AND N. AVDALOVIC 



two months later is well tolerated, although a simultaneously 

 transplanted graft from the third party is destroyed. 



The breakdown of tolerance is followed by a "null period" 

 during which the formerly tolerant animal disposes of the second 

 graft from the parabiotic partner by the first-set reaction. 



Tolerant parabionts are chimeras. A new cytological method 

 for identification of donor cells in tolerant chimeras is introduced. 



The competitive replacement of the immune system of the host 

 by that of the donor has been proposed as a possible mechanism 

 responsible for induction of tolerance following short-term 

 parabiosis. 



Acknowledgements 



We are perpetually in debt to Dr. S. Ohno, Dr. J. TrujiUo and Mrs. 

 Patricia A. Ray of the Department of Experimental Pathology, City of Hope 

 Medical Center, Duarte, CaHfomia for their continued help and advice. We 

 are also grateful to Mrs. Gjurgja Horvat for her assistance in this work. 



REFERENCES 



Barnes, D. W. H., Ford, C. E., Ilbery, P. L. T., and Loutit, J. F. (1958). 



Transplant. Bull, S9 loi. 

 Biggs, P. M., and Payne, L. N. (1959). Nature (Lond.), 184, 1594. 

 BiLLiNGHAM, R. E., and Brent, L. (1959). Phil. Trans. B, 242, 439. 

 BiLLiNGHAM, R. E., Brent, L., and Medawar, P. B. (1953). Nature [Lond.), 



172, 603. 

 Cock, A. G., and Simonsen, M. (1958). Immunology, 2, 103. 

 Doak, S. M. a., and Koller, P. C. (1961). Transplant. Bull, 27, 444. 

 DoRiA, G., Goodman, J. W., Gengozian, N., and Congdon, C. C. (1962). 



/. Immunol. 88, 20. 

 EiCHWALD, E. J., LusTGRAAF, E. C, FusoN, R. B., and Weissman, I. (1961). 



Proc. Soc. exp. Biol. (NY.), 106, 441. 

 FiNERTY, J. C. (1952). Physiol. Rev., 32, 277. 

 Hasek, M. (1953). Csl. hiol, 2, 265. 



Main, J. M., and Prehn, R. T. (1955). J. nat. Cancer Inst., 15, 1023. 

 Medawar, P. B. (1944)7. Anat. (Lond.), 78, 176. 

 Nakic, B. (1962). sih Int. Tissue Transplantation Conf. New York: N.Y. Acad. 



Sci. (in press), 

 Nakic, B., Nakic, Z., and Silobrcic, V. (i960). Nature [Lond), 186, 322. 



