Between March, 1947, and December,1953, 359 cottontails were examined for evidence of fibromas at the Patuxent Research Refuge, Laurel, Md. No tumors were observed on the rabbits until December, 1950, when 3 of 16 shot animals had fibromas. Twelve rabbits of 70 examined in 1951, had tumors, and no tumors were found on 37 examined in 1952 and 29 examined in 1953. Neutralization tests, using Shope (OA) fibroma virus, were positive on sera from 6 of 36 rabbits trapped between July and December, 1951; one of two rabbits tested in 1952 showed evidence of antibodies; and 2 of 9 tested in 1953 were positive. Neither fibromas nor circulating antibodies were found in 70 cottontails trapped in January and February, 1953, in Rockville, Maryland and over 30 cottontails obtained from dealers in Kansas and Arkansas. No lesions developed on several species of mammals inoculated with a strain of the fibroma virus isolated from a wild cottontail captured on the Refuge. These findings were interpreted as indication of occurrence of an epizootic during the period December, 1950 through December, 1951. Although the local cottontail population showed a marked drop following this epizootic, available data were insufficient to determine whether or not fibroma virus was a contributing factor.
