The relationship of intracytoplasmic type A particles, precursor particles of type B retroviruses, to microtubule organizing centers and to the mitotic apparatus have been studied electronmicroscopically in hamster and murine cell lines in various stages of mitotic arrest by Colcemid and vincristine sulfate. It was shown that the migration of these precursor particles from the cytoplasm to the kinetochore region of chromosomes, occurring in early metaphase, is a function of the inhibition of microtubule formation at the pericentriolar cytocentrum and of the attachment of spindle fibers at one sister kinetochore plate of chromosomes. The association of these type A particles with the mitotic apparatus during Colcemid arrest is reversible by removal of the inhibitor and is inversely proportional to the reattachment of spindle fibers and to the reformation of microtubules. The active participation of microtubules and cytoskeletal proteins in the transport and maturation of type B oncornaviruses is strongly suggested by these findings. Their implications, as to a possible epigenetic mode of transmission of these viruses as well as to the induction of the transformed state are discussed.