The intravenous inoculation into chick embryos 9-15 days of age of homologous tissue suspensions (brain; whole embryo; chorio-allantoic membrane; yolk-sac) results in the production of disseminated hemorrhages and rapid death. Such materials inoculated into 18-day embryos fail to produce hemorrhage but induce intravascular clotting and death. No abnormal reactions follow their intraven. inoculation into mice. Tissue suspensions prepared from various organs of the mouse produce no effect in chick embryos, although in the mouse they cause ataxia, convulsions and, occasionally, death. The thromboplastic activity of chick embryo and mouse tissue suspensions when added to avian and mammalian plasma exhibit specificity parallel to that of their toxic effect. It is suggested therefore, that the mechanism of the latter phenomenon probably consists of coagulation of the blood in vivo. Because of the failure, however, of the plasma of younger chick embryos to form typical coagula, this hypothesis cannot be accepted without reservation. The relevancy of these observations to serial passage of infected tissue suspensions in chick embryos, to tissue culture technics involving chicken plasma and to expts. on anaphylaxis is pointed out.