The experiments in this paper show that testicle extract causes India ink particles and those of Prussian blue to spread much more extensively through the intercellular spaces than similar suspensions made with Ringer's solution. Methylene blue inoculated intravenously localizes more extensively in areas previously injected with testicle extracts than in control areas receiving injections of tissue extracts without enhancing power. Kidney extracts have this property to a less degree, whereas spleen extracts and blood serum are devoid of it. The spreading power of extracts is destroyed by heating at 60°C. for 30 minutes, as is also the power to enhance infections. The precise mode of action of the Reynals factor is not known, but the results of the experiments here presented suggest that it may depend at least in part on the property whereby testicle extract increases the spread of injected material and alters the permeability of tissue cells. It is not inconceivable that changes in permeability facilitate the passage of vaccine virus through the endothelial cells of the blood and lymph vessels, and lead to the generalized vaccinia which is of frequent occurrence in the reported results (20).

It has been shown that fluids and suspensions of inert particles are spread by the extract.B. tetanus and B. coli exotoxins and trypsin were not enhanced at all in their action despite the fact that they were spread through a more extensive area in the tissues. Viruses, on the other hand, are markedly influenced and in this respect resemble bacteria, not toxins and enzymes. It appears probable that a definite capacity for multiplication on the part of an injected substance is required if its pathogenic effects are to be enhanced. It may be concluded tentatively that the enhancing power of the testicle extract may depend on that property which not only spreads the injected material through a larger area but renders the tissue cells more easily penetrable by the agents.

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