In vitro cultivation of the mouse mammary tumor virus: Correlation of infectivity and morphology with radioisotope studies

Organ and cell cultures of BALB/cfC3H mammary tumor tissue release infectious MTV into the medium. Not only were bioassays of culture fluid for MTV positive, but also radioisotope‐labelled MTV‐RNA and MTV B‐particle antigenicity could be detected in the culture fluid. In addition, type B virus particles were found budding from the cell membranes. Bioassays of control MTV‐free BALB/c tumor cultures were negative, as were radio‐chemical, immunological and morphological studies. Thus, infectivity could be correlated to the presence of labelled B particles in the cultures.