The activity of a number of substituted alkylaminoanthraquinones was compared in transplanted murine tumor systems including P388 and L1210 leukemias, B16 melanoma, and colon carcinoma 26. The structure-activity relationships among this class of compounds are discussed. Several derivatives had very high antitumor activity in several tumor systems. Two of the most active derivatives, namely, 1,4-bis(2-[(2-hydroxyethyl)amino]ethylamino)-9,10-anthracenedione (bisalkylAAD) and 1,4-dihydroxy-5,8-bis(2-[(2-hydroxyethyl)amino]ethylamino)-9,10-anthracenedione (dihydroxybisalkylAAD), which had curative activity in the above-mentioned tumors, were compared in considerable detail. DihydroxybisalkylAAD showed distinct advantages over bisalkylAAD in several tumor systems and is tenfold more potent with respect to effective dose range. This last difference is important for two reasons. First, these aminoanthraquinones are strong and persistent blue dyes and the administration of lower doses would minimize a potential cosmetic drawback of these compounds. Second and most important, iv administration of dose levels of bisalkylAAD which are within the therapeutic dose range on intermittent dose schedules produced convulsions and immediate death. IV administration of dihydroxybisalkylAAD also caused acute toxicity, but, because of its increased potency relative to antitumor activity and delayed toxicity, this acute toxicity was apparent only at doses well above the therapeutic dose range. All of the aminoanthraquinones evaluated, regardless of their activity as antitumor agents in vivo, proved to be potent inhibitors of DNA and RNA synthesis in vitro and bound strongly to DNA as evidenced by deltaTm values (deltaTm = upward shift in DNA melting temperature). Thus, the strong antitumor activity of aminoanthraquinones would appear to be due to some mechanism other than, or in addition to, DNA binding and inhibition of nucleic acid synthesis.