The RNAs of oncornaviruses may be separated into two major components (Duesberg, 1968; Biswal & Benyesh-Melnick, 1969; Erikson, 1969), a rapidly sedimenting 60 to 70 S component and a slowly sedimenting 4 S component. Under conditions of mild denaturation, the 60 to 70 S RNA dissociates to subunits of 30 to 40 S (Duesberg, 1968; Biswal & Benyesh-Melnick, 1969; Erikson, 1969) and a distinct 4 S subunit (Erikson & Erikson, 1971; McCain, Biswal & Benyesh-Melnick, 1971). In this report we show that the 69 S RNA of murine sarcoma-leukaemia virus (MSV-MLV) is composed of at least three physico-chemically distinct subunits.