The intravenous or the oral administration of a pine cone extract of Pinus parviflora Sieb et Zucc (Fr VI) and the related synthetic agent (DHP-FA) to lactating SHN mice prevented an increase of milk levels of mouse mammary tumour virus (MMTV) from day 7 to day 14 of lactation. Furthermore, Fr VI decreased the MMTV level at the 2nd lactation compared to the 1st lactation. This is the first report on the inhibition of milk MMTV of mice in the in vivo system.