Occurrence of Mason-Pfizer monkey virus in healthy rhesus monkeys.

The Mason-Pfizer monkey virus (M-PMV) was originally isolated from a spontaneous mammary adenocarcinoma of a female monkey. Searching for evidence of M-PMV in healthy rhesus monkeys has led to the recovery of similar isolates from cultures derived from normal mammary, fetal, and placental tissues. The viral morphology as well as the process of virus formation observed with the new isolates are identical to the ones reported for M-PMV where the doughnut-shaped type-A particle is incorporated into the viral nucleoid at the time of budding from the cell surface. The major internal protein, MW. 27,000 daltons, of the prototype M-PMV and those of new isolates from normal monkeys were found to be serologically identical. Although the presence ofM-PMV or its antigens has appeared to be of common occurrence in tissue specimens, sera from normal monkeys failed to show any specific reactivity against M-PMV by microimmunodiffusion or immunofluorescence tests.