Several investigators have shown that mouse sarcoma virus, moloney strain (MSV-m), induces foci of altered cells in tissue culture of mouse or rat cells (Hartley & Rowe, 1966; Fischinger, Messore & O'Connor, 1967; Ting, 1966; Bernard, Boiron & Lasneret, 1967). The focus-forming capacity of MSV-m is now utilized for titrating the virus (Hartley & Rowe, 1966). Interferon has been proved to inhibit both the multiplication of oncogenic viruses and the cellular transformation induced by these viruses in vivo and in vitro. This inhibitory effect concerns DNA viruses such as polyoma or SV 40 (Atanasiu & Chany, 1960; Allison, 1961; Todaro & Baron, 1965) as well as RNA viruses such as Rous sarcoma virus (Strandström, Sandelin & Oker-Blom, 1962; Bader, 1962; Traub & Morgan, 1967) and murine leukaemia viruses (Gresser et al. 1966). We report here the inhibitory action of preparations of interferon on the development of foci of altered mouse cells induced in vitro by MSV-m.