A series of tetracyclic quinoline- and quinoxalinecarboxamides were prepared, and their cytotoxicities were evaluated in a series of murine human tumor cell lines. Most of the quinoline derivatives were prepared by an adaptation of the Pfitzinger synthesis, followed by thermal decarboxylation and coupling with N,N-dimethylethylenediamine via a mixed anhydride method using isobutyl chloroformate. The quinoline analogues showed cytotoxicities broadly similar to those of the known tricyclic acridine-4-carboxamide mixed topoI/II inhibitor DACA, with thieno and indeno analogues being the most active. They showed little decrease in potencies against the Jurkat human leukemia topo II-resistant lines JLA and JLC, suggesting their cytotoxicity does not result primarily from inhibition of topo II. The quinoxaline analogues had more varied IC50 values, being on average less cytotoxic than the quinoline derivatives, but appeared to have a similar mode of action. Overall, this new class of compounds appear to be mixed topo I/II inhibitors, up to 3-fold more cytotoxic than DACA in the human leukemia cell lines studied, with in vivo activity in colon 38 comparable to that of DACA and doxorubicin.