Effects of amsacrine in combination with other anticancer agents at ID80 were evaluated by cell growth assay using a human T-cell leukemia cell line (MOLT-3). The data were analyzed with the aid of an improved isobologram, using the concept of an envelope of additivity. A supra-additive effect was observed for amsacrine in combination with cytosine arabinoside and mitoxantrone. An additive effect was observed in its combinations with bleomycin, CPT-11, cisplatin, daunorubicin, doxorubicin, etoposide, 5-fluorouracil, homoharringtonine, mitomycin C, or vincristine. 6-Mercaptopurine had an additive effect with amsacrine at ID80 but a sub-additive to protective effect at ID90. A sub-additive to protective effect was shown for amsacrine in combination with methotrexate. These data suggest that cytosine arabinoside and mitoxantrone are the best of the anticancer agents we studied for use in combination with amsacrine. Bleomycin, cisplatin, CPT-11, doxorubicin, cytosine arabinoside, homoharringtonine, mitomycin C, and vincristine also yielded favorable results when administrated simultaneously with amsacrine. Simultaneous administration of amsacrine with 6-mercaptopurine and methotrexate is not appropriate. If amsacrine is combined with 6-mercaptopurine and methotrexate, other suitable schedules should be explored. These results may provide a rationale for the design of clinical protocols combining amsacrine with other anticancer agents.