The effect of mouse L-cell interferon on bovine papillomavirus type 1 (BPV-1) transformation of murine cells was examined. Mouse interferon reduced the level of BPV-1-induced transformation of mouse C127 cells by 95%. Long-term treatment of established BPV-1-transformed mouse cell clones with mouse L-cell interferon led to a decrease in the average number of the plasmid viral genomes present in these cells to 1/3 to 1/8. Although revertant lines could not be isolated from these lines in the absence of treatment with interferon, flat revertants were easily selected from two independent clonal transformed lines carried for 60 generations in the continued presence of 200 units of interferon per ml. These flat revertants had the biological characteristics of nontransformed C127 cells and could be retransformed by BPV-1. Southern blot hybridization failed to detect BPV-1 DNA in any of eight independent revertant lines examined under conditions that could detect 0.2 copies per cell. We conclude that interferon treatment has resulted in a selective reduction of the amount of extrachromosomal BPV-1 DNA in transformed cells and has cured some treated cells completely of their viral DNA.