Abstract
The morphogenesis of a type 1 foamy agent (strain MK5) propagated in HEp2 cells was examined using ultrathin sections. The virus consisted of a nucleoid (350 Å) surrounded by two shells 600 Å and 900 Å in diameter. The inner shell and the nucleoid (together making a spherical internal component) appeared in the cytoplasm and by budding became enveloped by a morphologically altered cell membrane which bore projections. The staining properties of the virus depended on the method of fixation. Inclusions seen in infected cells are described. Strain MK5 was indistinguishable morphologically from a strain of type 2 foamy virus.