An inactivated and lyophilized preparation of a low virulence strain (Su) of Streptococcus pyogenes (group A) was designated OK-432. When 2- and 5-month-old AKR mice were inoculated im with OK-432 twice weekly throughout their life-spans, spontaneous leukemias occurred later and at a lower incidence than in control groups. By virus neutralization and cytotoxicity tests and by immunoelectron microscopy, antibodies against virus and cell-surface antigens of transplanted AKR leukemia were not detectable in sera of nonleukemic mice of any group. Whereas sera from mice treated with OK-432 were the only positive for interferon, viremia was clearly demonstrated in control groups by reverse transcriptase assays of the plasma.
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Aoki et al. (1976) studied this question.