Renal adenocarcinomas of the leopard frog, Rana pipiens, collected in Kandiyohi County, Minn., were fixed for electron microscopy prior to and 4, 7, 13, and 16 days after the entry of host frogs into lakes for overwintering. Fifteen tumors were studied. No herpesviruses were detected in 4 tumors of prehibernating frogs. Two tumors of frogs autopsied on Day 4 of overwintering did not show viruses. Mature herpesviruses were observed in 2 tumors of frogs that had been in lake water for 7 days. The temperature of the water was 9° on Day 4 and 5° on Day 7. Three of 7 tumors from frogs that had been in the lake for more than 7 days did not contain viruses detected with the electron microscope. All tumors examined from frogs that had been in lake water for more than 1 month contained viruses.

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This study was supported by Grant 990-CT from the Damon Runyon Memorial Fund for Cancer Research, Inc., New York, N. Y., and Grant GB-29482 from the National Science Foundation.

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