The [mouse] Madison 109 lung carcinoma (M109) was evaluated as a model for the screening of antitumor agents. Thirty-five drugs with established antitumor activity were assayed in mice implanted i.p. or s.c. with M109. Depending on the mode of tumor implantation, drugs representing those affecting nucleic acids (through binding, intercalating or inducing single-strand breaks) various alkylating agents, mitotic inhibitors, antimetabolites and immunodulators were able to inhibit the growth of s.c. M109 or to extend the lifespan of mice given M109 i.p. The i.p. implanted tumor was markedly affected (median survival time of treated/control mice, .times. 100: .gtoreq. 200% with occasional cures) by doxorubicin, mitomycin C, 10-hydroxy-camptothecin and dihydroxyanthraquinone. The s.c. implanted tumor was markedly affected (treated-control of .gtoreq. 12 days with regard to median time to grow 1 g tumors) by bleomycin and an analog, talisomycin, and by 6-thioguanine. The M109 was responsive to many different classes of clinically active agents and can serve as a useful tool in the screening of drugs with such potential. It may be particularly useful in screening analogs of camptothecins, nitrosoureas, bleomycins and mitomycins and for evaluating anthraquinones, anthracyclines, mitotic inhibitors, antimetabolites and immunomodulators.