Observations on the details of ultrastructure of a series of type c viruses.

High-resolution electron micrographs of a series of budding type C virions of murine leukemia virus (Moloney), murine leukemia virus (Rauscher) grown in mouse or human cells, and cat leukemia virus grown in cat or human cells show species differences. They all differ from the virions growing from ESP-I cells, the human cell line that produces type C virus. The difference is in the thickness and interrelationships of the intermediate layer common to all type C viruses. The differentiation of the intermediate layer is considered not to be under host-cell control but to be viral-genome directed.