Summary and Conclusions: By the intravenous administration of trypan blue, the immediate skin reaction to a homologous antigen may be demonstrated in serum treated rabbits and guinea pigs in the usual testing for direct, passive and reversed passive sensitization in both animals. The same type of reaction may be shown to an injection of sera of sensitized animals, of sera showing so-called primary toxicity for the guinea pig, and to very high dilutions of histamine. It is believed that the infiltration of the dye into these reaction sites is, at present, to be looked upon as no more than a sort of indicator of the presence of an edematous change which is an expression of a toxic action upon the capillary endothelium.