Testicle extract possesses a notable power to increase red cell fragility. Testicle extracts of the rat, rabbit and guinea pig all exhibit this property, which is most pronounced with the extract from the rat and least so with that from the guinea pig.

Splenic extract does not possess the property, or possesses it only to a very slight degree.

These results support the hypothesis that the factor in testicle extract which enhances infections (the Reynals factor) does so by altering the permeability of the host tissue.

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