Thirty‐two early‐passage human embryonic cultures were examined in the electron microscope and one of them, derived from a muscle tissue, contained two budding and four mature type‐C virus particles. Of the 15 original embryonic tissues from five embryos, only one showed the presence of a virus‐like particle. It is of interest to note that the tissue specimens which contained virus‐like particles originated from the same embryo.

Type-C virus particles are synthesized by cells from a number of human embryonic cultures infected with the Rauscher murine leukemia virus ( R V ) . These particles have the characteristic morphology of the murine leukemia viruses. In some cultures, t y p e 4 particles were also observed budding into the cisternae of rough endoplasmic reticulum of a few cells. Virus particles in the vicinity of such cells were of '' immature" type. I n contrast, particles in the vicinity of those cells which did not synthesize intracisternal type-C particle were of the mature" type. It is not known if the " mature" intracisternal particles have the morphology of the '' mature '' extracellular t y p e 4 particles.

SummarySeveral types of cell strains and established cell lines of simian and human origin failed to demonstrate loci of altered cells following infection with the Mason-Pfizer monkey virus (M-PMV). However, most diploid cultures, after infection, lived longer and displayed the ability to grow in soft agar medium. The number of cell colonies developing in the soft agar was directly proportional to the amount of virus added to the culture. Two types of cell colonies were isolated from soft agar after infection of monkey foreskin cells with M-PMV. One had characteristic fibroblastic morphology, and the other showed an epithelioid cell phenotype. The ratio of fibroblastie colonies to epithelioid colonies was in excess of 20 : 1. The epithelioid cultures displayed a complete lack of topoinhibition, formed three dimensional cellular dome structures, and demonstrated significant karyotypic alterations. Fibroblastic sublines, on the other hand, did not show formation of domes but presented some lack of topoinhibition. The majority of cells in fibroblastie sublines also continued to show a normal rhesus chromosome complement. Although both epithelioid and fibroblastie transformed cell types produced intracellular M-PMV antigen and virus particles, the infectious virus titers were significantly different. The noninfectious virus preparations recovered from some of the fibroblastic sublines contained a high percentage of aberrant forms of M-PMV.

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