Induction of type B virions to bud into cytoplasmic vacuoles in a mammary tumor of an X-ray- and urethan-treated X/Gf mouse.

Electron microscopic studies of thin sections of a mammary carcinoma that developed in a female mouse of the syngeneic X/Gf low-cancer strain after treatments with X-rays (3 X 100 rads weekly) followed by 8 weekly i.p. injections of urethan (1 mg/g body weight) revealed extensive budding of B particles into cytoplasmic vacuoles and numerous characteristic B virions in the vacuolar lumens. Simultaneous budding of B particles into cytoplasmic vacuoles and budding of B particles at plasma membranes of the same cancer cell were also noted (figs. 2 to 6).