An epithelial tumor with acidophilic intranuclear inclusions frequently occurs in the kidneys of leopard frogs. This tumor usually has the appearance of an infiltrating and destructive adenocarcinoma, which, when large, not uncommonly metastasizes; less often it is more orderly and adenomatous.
When inoculated as living fragments or cell suspensions into the lymph sacs, the cranial cavity, or the abdomen, no significant local growth results and the implanted material is resorbed. However, in approximately 20 per cent of the frogs surviving inoculation for more than 6 months, tumors develop in the kidney, which are like the "spontaneous" neoplasms. The incidence far exceeds that in the controls.
Desiccated and glycerinated tumor injected into the abdomen gives the same result as inoculation with living tumor; in somewhat over 20 per cent of animals surviving more than 6 months kidney tumors occur.
In alien species of frogs, no such tumors are produced by inoculation either with living or with desiccated tumor.
These experiments indicate the probability that the kidney tumor of the leopard frog is caused by an inclusion-forming, organ-specific virus.