The presence of an inhibiting substance in the chicken tumor is shown by the fact that a desiccate of the tumor is more active after it has been washed two or three times with water, and that an extract of the tumor is more potent after some factor is removed by adsorption on aluminum hydroxide.

When the tumor-producing factor in an extract of a slow-growing tumor has been destroyed by heating at 55°C. it is found to have the property of neutralizing a highly active tumor extract. This inhibiting property is destroyed by heating over 65°C.

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