Oximidine III is an antitumor antibiotic that selectively inhibits the growth of rat fibroblast 3Y1 cells and induces mutations in various tumor genes. The inhibitory effects of Oximidine III on vH-ras-3Y1, v-src-3Y1 cells, and normal 3Y1 cells have IC50 values of 14 nM, 4.5 nM, and 140 nM, respectively. Additionally, Oximidine III arrests cells with RAS or SRC mutations in the G1 phase of the cell cycle and increases the expression of p21WAF1.