PRIMARY SERUM TOXICITY AS DEMONSTRATED BY THE CHICKEN EMBRYO.

1. The 3 day old chicken embryo removed from its shell is a suitable test object for the demonstration of primary serum toxicity. Addition of normal rabbit type sera as well as Forssman antiserum causes the vascular network to contract and the embryo sinks in the yolk and dies. 2. Only sera of animals of the so called rabbit type produce this phenomenon. Sera of the guinea pig type are ineffective. 3. Heating to 51 degrees C. destroys the complement content of normal human serum as also its effectiveness to produce the vascular phenomenon. 4. Up to the present it has not been possible to reactivate heat-inactivated normal serum by the addition of complement, while inactivated Forssman antiserum can be easily reactivated. 5. The vascular phenomenon of the chicken embryo is produced not only by the addition of a mixture of Forssman antiserum and complement but also by separate addition of both components. 6. Guinea pig type sera, containing dissolved Forssman antigen, are not only ineffective but actually exert an inhibitory influence on effective rabbit type sera as well as on Forssman antiserum.