Rats classified as reactors to dextran respond to fresh egg white and to ovomucoid both intraperitoneally and intravenously; however, larger intravenous doses of egg white and of ovomucoid are required to produce a response in non-reactor rats. The active component in fresh egg white is thermostable. The reaction produced by one sample of dextrin in both reactor and non-reactor rats is similar both intraperitoneally and intravenously respectively. Repeated intraperitoneal does of egg white or dextran into reactor rats produce refractoriness to a further intraperitoneal challenge of either of these agents, and to an intraperitoneal challenge of dextrin; such doses, however, reduce the response of intravenous egg white and dextran but do not modify that of intravenous dextrin. Repeated intraperitoneal doses of dextrin into reactor rats produce refractoriness to a further intraperitoneal challenge of dextrin but only reduce the responses of intraperitoneal egg white and dextran; the intravenous response to dextran is also reduced by such treatment. Repeated intraperitoneal doses of dextrin into non-reactor rats produce refractoriness to intraperitoneal dextrin and to intravenous egg-white but not to intravenous dextrin. The new definition of non-reactivity in Wistar albino rats is the failure to respond to intraperitoneal and intravenous dextran and only to intraperitoneal egg white.