Flavone-8-acetic acid (FAA), a new antitumor agent currently undergoing clinical trial, fails to inhibit the growth of early stage Lewis lung (LL) tumors growing in the lung. However, the growth of advanced subcutaneous tumors, arising from inoculation of either the original in vivo LL line or a tissue culture-adapted cell line (LLTC) derived from the LL line was delayed significantly by FAA treatment. Comparison, by clonogenic survival assays, of the cytotoxic effect of FAA on LLTC cells demonstrated that most cell killing occurred between 2 and 8 hours following in vivo exposure but occurred to a much lesser extent and at later times following in vitro exposure. FAA was inactive against LLTC cells growing in vivo in diffusion chambers, suggesting that a host cellular component was necessary for activity. FAA was found to induce hemorrhagic necrosis in the advanced LL tumors, as well as in a number of human tumor xenografts growing in athymic mice. The human cell lines from which the xenografts were derived, as well as the LL tumor lines and P388 leukemia lines, were inhibited by FAA in vitro. However, the ranking of FAA activity in vivo did not parallel that observed in vitro. Together, these observations strongly suggest that FAA has an indirect mode of antitumor action.