The growth of a mouse leukemia virus in an established mouse cell line was examined after the line became contaminated with an unidentified Mycoplasma species. The contaminated cultures grew well in small plastic cultures dishes, but they could not be propagated in larger roller bottles unless the growth medium was changed frequently. Cells from Mycoplasma-contaminated and Mycoplasma-free cultures were exposed to 3H-labeled uridine for 24 hr. Culture fluids were harvested 2 or 24 hr after labeling and purified by centrifugation through discontinuous sucrose gradients. Considerably less uridine-3H-labeled virus was recovered from supernatant fluids of Mycoplasma-contaminated cultures than from Mycoplasma-free cultures. Equilibrium sedimentation in sucrose gradients of uridine-3H-labeled material from culture supernatants of contaminated cultures produced 3H peaks at buoyant densities of 1.20 to 1.24 and 1.16 to 1.18 g/ml. Virus titers in culture fluids from Mycoplasma-contaminated cultures were greatly reduced as judged from viral interference tests. The viral RNA was degraded to low-molecular-weight species when virions were harvested 2 to 24 hr after labeling of Mycoplasma-contaminated cultures.

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Supported by USPHS Grants CA04311 from the National Cancer Institute and HD04583 from the National Institutes of Child Health and Human Development.

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