In order to isolate and analyze histone fractions of Shope papolloma virus (SPV) -induced papillomas and papilloma-transformed cancer developed in adult domestic rabbits, a series of experiments were conducted in this study and the results are summarized as follows:
1. Inoculation of SPV into the depilated back skin of New Zealand white rabbits resulted in the development of typical papillomas at the site of virus inoculation. The virus inoculum, SPV, was prepared from papilloma tissues induced in cottontail rabbits by the inoculation of SPV and has been kept in 50% buffered glycerin at 4C for almost 20 years, and this study firmly established that SPV and its tumor-forming activity is extremely stable under the conditions of the storage.
2. As expected, spontaneous regression of tumors occurred in a high proportion of rabbits carrying, SPV induced papillomas by 8 weeks postinoculation. Of 4 rabbits in which spontaneous regression of papillomas did not occur, there observed no development of cancerous transformation from the papillomas during the prolonged period of observation up to 30 weeks following the SPV inoculation.
3. Development of solid immunity against papilloma induction by rechallenbe with SPV was evident among the rabbits which had spontaneous regression of papillomas that were induced following the primary inoculation of SPV.
-4. Attempts were unsuccessful to demonstrate the presence of the tumor-specific antigens: and SPV-related antigens in the tissues of "old" papillomas by double agar gel diffusion, counterimmunoelectrophoresis and indirect fluorescent antibody technic.
5. Electron micrographs of "fresh and "old" papillomas failed to reveal the presence of mature SPV virions or virus particles in the tumor tissues examined.
6. Histone fractions, i.e., H 1, H 2a, H 2b, H 3 and H 4, were successfully isolated from "fresh" and "old" papillomas and from normal rabbit skin tissue, and the fractions were characterized by amino acid analysis and by polyacrylamide disc gel electrophoresis. The major findings of histone studies include the followings
i) The yields of whole histories were 7. 348 mg/g of normal rabbit skin, 14. 091 mg/g of "fresh" papil-
loma and 9. 903 mg//g of "old"´ papilloma tissue, respectively:
li) The yields of DNA were 6. 252 mg/g of normal rabbit skin, 16.772 mg/g of "fresh" papilloma and
10.988 mg/g of "old" papilloma tissue, respectively.
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