New type B retrovirus isolates associated with kinetochores and centrioles of the host cell.

Two new virus isolates, M432 and M832, obtained from the Southeast Asian mouse have been characterized morphologically with respect to their composition and intracellular assembly. The mature virions resemble in certain respects the type B murine retroviruses. The new isolates, however, have an intracellular precursor, a type A particle, closely associated with the mitotic apparatus. The intracellular transport of the type A particles to the cell surface, where they are released by budding, is closely associated with the microtubule system of the cell.