Five anthracycline derivatives, i.e. doxorubicin, epirubicin, pirarubicin, aclarubicin and a new fluorinated anthracycline derivative (ME2303), were tested for antitumour activity in mice with hepatic neoplastic nodules of Lewis lung carcinoma and colon adenocarcinoma 26. Intravenous administrations of pirarubicin and ME2303 on day 4 or days 4, 8 and 12 in mice with hepatic neoplastic nodules of Lewis lung carcinoma rendered more than 50% of mice tumour-free over wide ranges of nontoxic doses, whereas a few mice were cured by treatment with doxorubicin and no mice were cured by treatment with epirubicin or aclarubicin. Moreover, when ME2303 was administered at 50 mg kg-1 on days 7, 11 and 15 to six mice bearing more advanced hepatic tumours, five were cured, while pirarubicin and doxorubicin never achieved cure. Furthermore, in mice with hepatic neoplastic nodules of colon adenocarcinoma 26, ME2303 also showed a marked antitumour effect compared to pirarubicin or doxorubicin. Two or three injections of ME2303 starting from day 7 conferred a greater antitumour effect than did more fractionated or single-dose regimens.