Some unusual findings of the murine mammary tumor virus (MuMTV) are described in cells of BALB/cfC3H mice mammary tumors: (1) a conspicuous endocellular production of B particles by budding of A particles into small and large cytoplasmic vacuoles, into intracytoplasmic lumina, and possibly into vacuoles of secretion; (2) an extracellular production of B particles (in addition to the classic budding via microvilli) by opening at the cell surface of intracytoplasmic lumina containing mature and immature B particles and possibly of small vacuoles containing immature B particles. Morphological data suggest a virus excretion also by the secretory pathway of the neoplastic cell. Golgi complex is often seen in association with the small type of cytoplasmic vacuoles, whereas the large vacuoles are often close to a particular type of dense body, possibly lysosomes. The occurrence of free B particles in the cytoplasm is also reported. The possibility that the observed features are not occasional but are aspects of the normal life of the MuMTV is suggested.