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Summary Normal serum from various animals has been shown to be toxic to lymphocytes and peritoneal exudate cells of various other animals. This toxicity could best be shown with fresh serum or in some cases only with fresh serum. Serum heated at 56°C for 30 min lost its activity in most instances. The activity lost upon storage or heating could not be completely restored by the addition of fresh homologous or autologous serum. Some particularly toxic combinations of serum-lymphocytes were: human-rabbit; human-mouse; rabbit-chicken; rabbit-rat; rat-chicken; chicken-rabbit; and chicken-guinea pig.

Summary Evidence has been found which indicates that tolerance to homografts in chickens is not completely individual specific (within one breed as well as across two breeds). A total of 71 embryonic chicks were injected with blood from embryonic chicks of a different breed and, 2 and 15 days after hatching, were grafted with skin from chicks other than the blood donor but of the same breed as the blood donor. A significant percentage of these chicks had grafts which survived longer than the grafts between control chicks not previously injected with blood. The possibility that the tolerance induced by blood of one chick to skin of another was due to similarity in antigenic makeup of the 2 donor chicks, since they were of the same breed, seems discounted by 2 experiments. First, it was shown that grafts among 2-week-old chicks of the same breed did not survive to any significant degree, thus indicating the genetic disparity among chicks of the breeds used. Secondly, when the nonspecific graft survived for a fairly long period, grafts between the blood and skin donor chicks often did not survive as long.

Summary Evidence is presented which indicates that some tolerance of skin homografts may be obtained in noninbred chickens by an embryonic injection of blood from a breed other than that of the skin donor. Sixteen AW chicks given blood (0.3–0.35 ml) from AW embryos on the 14th day of incubation and grafted 2 days after hatching with skin from 2-day-old GL and BR chicks showed a better survival of grafts than untreated AW and AL chicks whose skin was exchanged at 2 days. Thirteen WL embryos injected with NH embryo blood and 15 NH embryos injected with WL embryo blood and grafted 2 days after hatching with 2-day-old BR skin showed much greater tolerance than controls (BR skin on untreated NH and WL chicks). Possible explanations for such nonbreed-specific induction of tolerance are discussed.

Summary The serum of rabbits which have been homografted with skin was shown to be toxic to the lymph node cells of the skin donor. Fresh serum was toxic, and the activity of the serum declined rapidly upon standing overnight, freezing or heating at 56°C for 30 min. The loss of activity could be restored to a large extent by addition of fresh normal rabbit serum or fresh serum from the lymphocyte donor rabbit. Guinea pig complement exerted some inherent toxicity. The possibility that the detection of such cytotoxins may suggest a re-evaluation of the cellular basis of homograft immunity is discussed.