In vitro L1210 (V) cell lines contained abundant intracytoplasmic A-particles, numerous C-type particles, a small number of B-type particles, and occasional intracisternal A-particles. The intracytoplasmic A-particles were incorporated into both spiked (B-type) and smooth-surfaced (C-type) particles formed at the budding site. Both B-and C-type particles also developed by gradual accululation of neucleooid material. The particles, particularly the C-type, exhibited a wide range of densities. The cells showed strong surface immunofluoresence for both murine mammary tumor virus and Gross murine leukemia virus antigens and variable degrees of cytoplasmic immunoflurescence for the protein antigens (p1 to p6) of Rauscher leukemia virus. The cells, the culture supernatant, and the purified virus each gave positive reactions with murine mammary tumor virus and murine leukemia virus antisera by immunodiffusion. The viral particles failed to infect C57BL, C57BL/6 X DBA/2F1 (hereafter called BD2F1), BALB/c, Af,and RIIIf mice. Howver, the cells were highly tumorigenic in BD1F-1 mice, moderately tumorigenic in BALB/c mice, but not tumorigenic in C57BL, Af, and RIIIf mice.