Decrease in mammary tumour incidence in virgin C3H mice given interferon only while suckling.

A high proportion of females of the C3H strain of mice develop tumours of the mammary gland which are caused by mouse mammary tumour virus (MMTV) transmitted through the milk. We have examined whether administration of mouse interferon (IFN) to nursing mothers and/or their suckling offspring only during the period of nursing, can affect the incidence of tumours developing in these animals. In two separate experiments, animals receiving IFN by direct injection while suckling, and remaining virgin showed a marked and statistically significant decrease in tumour incidence. Mice receiving the same or a tenfold higher dose of IFN while lactating showed no such reduction in tumour incidence, even if they had also received IFN while suckling. The results suggest that IFN can affect the initial establishment of the MMTV infection in suckling mice sufficiently to delay tumour development provided the animals are not exposed to the hormonal stimulus of pregnancy and lactation.