Ruboxyl (RBX), a new nitroxyl derivative anthracycline, showed an interesting cytotoxic effect on in vitro established human cell lines and an antitumor activity in tumor-bearing animals. In this study, we further investigated the antitumor effect of this drug compared to Doxorubicin (DX) in four in vivo systems of experimental murine tumors. Our data demonstrate that RBX had little effect on the growth of primary tumor, and on the survival, in mice bearing Lewis lung carcinoma (3LL), while the drug showed a higher effect on the growth of B 16 melanoma. In both the experimental murine systems the activity of RBX was similar to that exerted by DX. As regards the leukemia models, RBX induced a significant increase on survival in mice bearing both L1210 or L5178Y leukemias. However, the effect on survival and the number of long-term survivors (LTS) were lower than that observed following DX treatment.