Bromodeoxyuridine (BrdU), whether administered in a 30-hr pulse of 30 mug/ml or continuously in low concentrations (1-3 mug/ml), significantly increased production of particles with the morphology of murine leukemia virus in a mouse melanoma (B16) cell line. Particles were very rare in control cells, detectable only by electron microscopy. By contrast, in many experiments with BrdU-treated cells the numbers of virus particles counted by electron microscopy increased over 100-fold, and other tests for murine leukemia virus (plaque assay and tests for group-specific antigens 1 and 3 and for Gross cell-surface antigens) became positive. All BrdU-treated cells, regardless of drug concentration or length of treatment, in addition to showing loss of both pigment and of piled-up morphology, were suppressed in tumorigenicity compared with the control cells. These effects were all reversible. A significant percentage of mice injected with BrdU-treated cells were protected against subsequent tumor formation when challenged with malignant control cells. The degree of protection conferred on the mice correlated well with the number of virus particles counted in the injected cells. There was also good correlation between the amount of cell-associated virus and the degree of suppression of malignancy for cells treated continuously with 1 mug of BrdU per ml, but not as good for cells treated for short periods with higher concentrations of BrdU.