Abstract
In BHK21 hamster cells a significant increase in density of intramembranous particles occurs in freeze-fractured plasma membranes after transformation by hamster sarcoma and polyoma viruses. A similar change has been observed in chick embryo cells infected and transformed by a mutant of Rous sarcoma virus thermosensitive for transformation, at both permissive and nonpermissive temperatures. There is also an increase in particle density in chick cells infected with the Rous-associated avian leukosis virus type 1. The newly appeared particles may represent the insertion of new proteins in hydrophobic regions of plasma membrane, in response to the action of oncogenic viruses.